Water-Based Coating Composition, Method for Forming Multi-layer Coating Film, and Article Having Multi-layer Coating Film

ABSTRACT

A water-based coating composition and method for forming a multi-layer coating film by which it is possible to form a coating film having excellent smoothness, image clarity, and chipping resistance in a 3C1B format. The composition is characterized by containing 7 to 60 mass % of an acrylic resin emulsion (a), 10 to 60 mass % of a film-forming resin (b), and 10 to 50 mass % of a curing agent (c), on the basis of the total mass of the solid resin content in the composition; the acrylic resin emulsion (a) having a core/shell structure where the core section contains, as a copolymer component, 0.1 to 10 mass % of a polymerizable unsaturated monomer having two or more polymerizable unsaturated groups in one molecule, on the basis of the total mass of monomer of the core section, and the hydroxyl value of the core section is 50 to 200 mgKOH/g, and the hydroxyl value of the shell section is 50 to 200 mgKOH/g.

TECHNICAL FIELD

The present invention relates to a water-based coating composition to beused as a water-based first pigmented coating material in a 3-coat,1-bake system in which a water-based first pigmented coating material, awater-based second pigmented coating material and a clear coatingmaterial are coated in that order on an article to be coated, and theobtained 3-layer multilayer coating film is simultaneously heat cured,as well as to a multilayer coating film-forming method and an articlewith a multilayer coating film.

BACKGROUND ART

As a method of forming coating films for automobile bodies, there iswidely employed a method of forming a multilayer coating film by a3-coat, 2-bake (3C2B) system in which an electrodeposition coating isformed on an article to be coated, and this is followed by “intercoatmaterial application, bake curing, base coat material application, clearcoating material application and bake curing”, in that order. In recentyears, from the viewpoint of achieving energy savings, it has also beenattempted to employ 3-coat, 1-bake (3C1B) systems that eliminate thebake curing step following application of the intercoat material,wherein electrodeposition coating is carried out on an article to becoated, followed by “intercoat material application, base coat materialapplication, clear coating material application and bake curing”, inthat order (PTL 1, for example).

However, with 3C1B systems there is an issue of proneness to layermixing between the intercoating film and base coating film, which canlower the smoothness and distinctness of image of the obtained coatingfilm.

Also, automobile coating films are expected to have excellent damageresistance (“chipping resistance”) against damage by rocks and the likeduring travel, and as water-based intercoat materials with excellentchipping resistance there have hitherto been proposed, for example,water-based intercoat material compositions comprising a specific talcand silane coupling agent (PTL 2, for example).

In PTL 3 there is described, as an intercoat material in a 3-coat,1-bake system, the use of a water-based intercoat material compositioncomprising a copolymer resin emulsion obtained by emulsionpolymerization of a monomer (a) including at least one type of monomerselected from among alkyl (meth)acrylate esters, and if necessary atleast one type monomer selected from the group consisting ofstyrene-based monomers, (meth)acrylonitrile and (meth)acrylamide, anacid group-containing polymerizable unsaturated monomer (b), a hydroxylgroup-containing polymerizable unsaturated monomer (c) and acrosslinkable monomer (d), the copolymer resin emulsion having a glasstransition temperature (Tg) of −50° C. to 20° C., an acid value of 2 to60 mgKOH/g and a hydroxyl value of 10 to 120 mgKOH/g, and a curingagent, whereby curing reactivity between the copolymer resin emulsionand the curing agent is increased, and a multilayer coating film withsatisfactory chipping resistance and water resistance and a goodfinished appearance can be formed.

CITATION LIST Patent Literature

-   PTL 1 Japanese Unexamined Patent Publication HEI No. 10-216617-   PTL 2 Japanese Unexamined Patent Publication No. 2003-253211-   PTL 3 International Patent Publication No. WO2004/061025

SUMMARY OF INVENTION Technical Problems

At the current time, however, it has not been possible to obtain acoating composition and multilayer coating film-forming method that arefully satisfactory in terms of all the properties of chippingresistance, water resistance, smoothness and distinctness of image.

It is therefore an object of the present invention to provide awater-based coating composition and a multilayer coating film-formingmethod that allow formation of a coating film with excellent smoothness,distinctness of image and chipping resistance, by a 3C1B system.

Solution to Problems

As a result of diligent research aimed at achieving the object statedabove, the present inventors have completed this invention upon findingthat, in a coating process for a multilayer coating film by a 3C1Bsystem, it is possible to form a multilayer coating film with excellentchipping properties, smoothness and distinctness of image by using asthe water-based first pigmented coating material a coating materialcomprising a specific acrylic resin emulsion (a), a coating film-formingresin (b) and a crosslinking agent (c).

In other words, the invention provides a water-based coatingcomposition, a multilayer coating film-forming method and an articlewith a multilayer coating film, as follows.

1. A water-based coating composition to be used as a water-based firstpigmented coating material (A) in a multilayer coating film-formingmethod in which the following steps (1) to (4) are carried out in order:step (1): a step of coating an article to be coated with a water-basedfirst pigmented coating material (A) to form a first pigmented coatingfilm, step (2): a step of coating the uncured first pigmented coatingfilm formed in step (1) with a water-based second pigmented coatingmaterial (B) to form a second pigmented coating film, step (3): a stepof coating the uncured second pigmented coating film formed in step (2)with a clear coating material (C) to form a clear coating film, and step(4): a step of bake curing the multilayer coating film formed in steps(1) to (3), wherein:

the water-based coating composition comprises 7 to 60 mass % of anacrylic resin emulsion (a), 10 to 60 mass % of a coating film-formingresin (b) and 10 to 50 mass % of a curing agent (c), based on the totalmass of the solid resin content in the water-based coating composition,

the acrylic resin emulsion (a) has a core-shell structure, the coresection comprising, as a copolymerizing component, 0.1 to 10 mass % of apolymerizable unsaturated monomer having two or more polymerizableunsaturated groups in the molecule, based on the total mass of themonomers of the core section, and

the hydroxyl value of the core section is 50 to 200 mgKOH/g and thehydroxyl value of the shell section is 50 to 200 mgKOH/g.

2. A water-based coating composition according to 1. above, wherein thecore section comprises, as a copolymerizing component, 30 to 90 mass %of a copolymerizable unsaturated monomer with a C4 or greaterhydrocarbon group, based on the total mass of the monomers of the coresection, the acid value of the acrylic resin emulsion (a) is 5 to 25mgKOH/g, and its glass transition temperature (Tg) is 20° C. or higher.

3. A water-based coating composition according to 1. or 2. above,wherein the coating film-forming resin (b) comprises at least one typeselected from among polyester resins (b1), water-soluble acrylic resins(b2) and urethane resins (b3).

4. A water-based coating composition according to any one of 1. to 3.above, wherein the curing agent (c) comprises an amino resin (c1) and/ora blocked polyisocyanate compound (c2), and the water-based coatingcomposition includes 5 to 30 mass % of an amino resin (c1) and/or 5 to30 mass % of a blocked polyisocyanate compound (c2), based on the totalmass of the solid resin content in the water-based coating composition.

5. A water-based coating composition according to any one of 1. to 4.above, wherein the blocked polyisocyanate compound (c2) comprises anactive methylene-type blocked polyisocyanate compound (c3) with at leastone type of blocked isocyanate group selected from the group consistingof blocked isocyanate groups represented by the following formula (I):

(wherein R¹, R², R⁴ and R⁵ each independently represent a C1-12hydrocarbon group and R³ represents a C1-12 straight-chain or branchedalkylene group),blocked isocyanate groups represented by the following formula (II):

(wherein R², R³, R⁴ and R⁵ have the same definitions as above),and blocked isocyanate groups represented by the following formula(III):

(wherein R², R³, R⁴ and R⁵ have the same definitions as above, and R⁶represents a C1-12 hydrocarbon group).

6. A water-based coating composition according to any one of 1. to 5.above, wherein the water-based coating composition includes, as apigment, titanium dioxide and/or barium sulfate in a total range of 50to 150 mass % based on the total mass of the solid resin content.

7. A multilayer coating film-forming method comprising the followingsteps (1) to (4):

step (1): a step of coating an article to be coated with a water-basedfirst pigmented coating material (A) to form a first pigmented coatingfilm,

step (2): a step of coating the uncured first pigmented coating filmformed in step (1) with a water-based second pigmented coating material(B) to form a second pigmented coating film,

step (3): a step of coating the uncured second pigmented coating filmformed in step (2) with a clear coating material (C) to form a clearcoating film, and

step (4): a step of bake curing the multilayer coating film formed insteps (1) to (3),

wherein the water-based coating composition as the water-based firstpigmented coating material (A) comprises 7 to 60 mass % of an acrylicresin emulsion (a), 10 to 60 mass % of a coating film-forming resin (b)and 10 to 50 mass % of a curing agent (c), based on the total mass ofthe solid resin content in the water-based coating composition,

the acrylic resin emulsion (a) has a core-shell structure, the coresection comprising, as a copolymerizing component, 0.1 to 10 mass % of apolymerizable unsaturated monomer having two or more polymerizableunsaturated groups in the molecule, based on the total mass of themonomers of the core section, and

the hydroxyl value of the core section is 50 to 200 mgKOH/g and thehydroxyl value of the shell section is 50 to 200 mgKOH/g.

8. The method according to 7. above, wherein the core section comprises,as a copolymerizing component, 30 to 90 mass % of a copolymerizableunsaturated monomer with a C4 or greater hydrocarbon group, based on thetotal mass of the monomers of the core section, the acid value of theacrylic resin emulsion (a) is no greater than 15 mgKOH/g, and its glasstransition temperature (Tg) is 20° C. or higher.

9. An article with a multilayer coating film, formed by the methodaccording to 7. or 8. above.

Advantageous Effect of Invention

With the water-based coating composition of the invention it is possibleto form a multilayer coating film with excellent smoothness,distinctness of image and chipping resistance by a 3C1B system.

DESCRIPTION OF EMBODIMENTS

The water-based coating composition of the invention is a water-basedcoating composition to be used as a water-based first pigmented coatingmaterial (A) in a multilayer coating film-forming method in which thefollowing steps (1) to (4) are carried out in order: step (1): a step ofcoating an article to be coated with a water-based first pigmentedcoating material (A) to form a first pigmented coating film, step (2): astep of coating the uncured first pigmented coating film formed in step(1) with a water-based second pigmented coating material (B) to form asecond pigmented coating film, step (3): a step of coating the uncuredsecond pigmented coating film formed in step (2) with a clear coatingmaterial (C) to form a clear coating film, and step (4): a step of bakecuring the multilayer coating film formed in steps (1) to (3), whereinthe water-based coating composition comprises 7 to 60 mass % of anacrylic resin emulsion (a), 10 to 60 mass % of a coating film-formingresin (b) and 10 to 50 mass % of a curing agent (c), based on the totalmass of the solid resin content in the water-based coating composition,the acrylic resin emulsion (a) has a core-shell structure, the coresection comprising, as a copolymerizing component, 0.1 to 10 mass % of apolymerizable unsaturated monomer having two or more polymerizableunsaturated groups in the molecule, based on the total mass of themonomers of the core section, and the hydroxyl value of the core sectionis 50 to 200 mgKOH/g and the hydroxyl value of the shell section is 50to 200 mgKOH/g.

[Acrylic Resin Emulsion(a)]

The acrylic resin emulsion (a) to be used in the water-based coatingcomposition of the invention has a core-shell structure including a coresection and a shell section, and is preferably produced by two-stagepolymerization in an aqueous medium. Also, in acrylic resin emulsion(a), the proportion of copolymer (I) composing the core section(hereunder also referred to as “core section copolymer (I)”) andcopolymer (II) composing the shell section (hereunder also referred toas “shell section copolymer (II)”) is preferably in the range of about10/90 to 90/10, as the solid mass ratio.

[Polymerizable Unsaturated Monomer (m) Forming Acrylic Resin Emulsion(a)]

There are no particular restrictions on the polymerizable unsaturatedmonomer used to form the acrylic resin emulsion (a), and for example,the following compounds with polymerizable unsaturated groups, (m-1) to(m-5), may be mentioned.

As used herein, “polymerizable unsaturated group” means an unsaturatedgroup that can participate in radical polymerization. Examples of suchpolymerizable unsaturated groups include vinyl and (meth)acryloyl.

Also, “(meth)acrylate” means “acrylate or methacrylate”, “(meth)acrylicacid” means “acrylic acid or methacrylic acid”, “(meth)acryloyl” means“acryloyl or methacryloyl” and “(meth)acrylamide” means “acrylamide ormethacrylamide”.

[Polymerizable Unsaturated Monomer with Two or More PolymerizableUnsaturated Groups in the Molecule (m-1)]

Specific examples for the polymerizable unsaturated monomer with two ormore polymerizable unsaturated groups in the molecule (m-1) includeallyl (meth)acrylate, ethyleneglycol di(meth)acrylate, triethyleneglycoldi(meth)acrylate, tetraethyleneglycol di(meth)acrylate,1,3-butyleneglycol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, 1,4-butanediol di(meth)acrylate, neopentyl glycoldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, methylenebis(meth)acrylamide, ethylene bis(meth)acrylamide, pentaerythritoldi(meth)acrylate, pentaerythritol tetra(meth)acrylate, glyceroldi(meth)acrylate, 1,1,1-trishydroxymethylethane di(meth)acrylate,1,1,1-trishydroxymethylethane tri(meth)acrylate,1,1,1-trishydroxymethylpropane tri(meth)acrylate, triallyl isocyanurate,diallyl terephthalate and divinylbenzene.

Also, the polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule (m-1) includescombinations of two or more different polymerizable unsaturated monomerswith mutually reactive functional groups (m-11). This is because acombination of two or more different polymerizable unsaturated monomerswith mutually reactive functional groups (m-11), by undergoing mutualreaction before copolymerization, during copolymerization and/or aftercopolymerization of the acrylic resin emulsion (a), has the samefunction as a polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule (m-1), i.e. thefunction of crosslinking the core section. The combination of thefunctional groups used may be any combination of functional groups thatare mutually reactive, but a combination of an acidic group and aglycidyl group, an amino group and a glycidyl group or a hydroxyl groupand an isocyanate group is preferred.

Specific examples of combinations of two or more different polymerizableunsaturated monomers with mutually reactive functional groups (m-11)include combinations of (meth)acrylic acid and glycidyl (meth)acrylate,(meth)acryloyloxyalkyl acid phosphates and glycidyl (meth)acrylate,(di)alkylaminoethyl (meth)acrylates and glycidyl (meth)acrylate andhydroxyalkyl (meth)acrylates and (meth)acryloyloxyalkyl isocyanate, anyof which may be used alone or in combination of two or more.

Also, the polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule (m-1) includescombinations of polymerizable unsaturated monomers with a reactivefunctional group and compounds with two or more functional groups thatcan react with the reactive functional group (m-12). This is because acombination of a polymerizable unsaturated monomer with a reactivefunctional group and a compound with two or more functional groups thatcan react with the reactive functional group (m-12), by undergoingmutual reaction before copolymerization, during copolymerization and/orafter copolymerization of the acrylic resin emulsion (a), has the samefunction as a polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule (m-1), i.e. thefunction of crosslinking the core section.

Examples of combinations of a polymerizable unsaturated monomer with areactive functional group and a compound with two or more functionalgroups that can react with the reactive functional group (m-12) includeglycidyl group-containing polymerizable unsaturated monomers andpolybasic acid components, and hydroxyl group-containing polymerizableunsaturated monomers and polyfunctional isocyanate compounds. Specificexamples include combinations of glycidyl (meth)acrylate and adipic acidand hydroxyalkyl (meth)acrylate and hexamethylene diisocyanate, whichmay be used alone or in combinations of two or more.

[Hydroxyl Group-Containing Polymerizable Unsaturated Monomer (m-2)]

Specific examples for the hydroxyl group-containing polymerizableunsaturated monomer (m-2) include monoesterified products of(meth)acrylic acid and C2-8 dihydric alcohols, such as 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl(meth)acrylate and 4-hydroxybutyl (meth)acrylate,ε-caprolactone-modified forms of the monoesterified products of(meth)acrylic acid and C2-8 dihydric alcohols, N-hydroxymethyl(meth)acrylamide; allyl alcohols, and (meth)acrylates havingpolyoxyalkylene chains with hydroxyl group molecular ends.

[Copolymerizable Unsaturated Monomer with C4 or Greater HydrocarbonGroup (m-3)]

The polymerizable unsaturated monomer with a C4 or greater hydrocarbongroup (m-3) is a polymerizable unsaturated monomer having a C4 orgreater straight-chain, branched or cyclic saturated or unsaturatedhydrocarbon group, excluding monomers having hydrophilic groups such ashydroxyl group-containing polymerizable unsaturated monomers. Specificexamples of such monomers include alkyl or cycloalkyl (meth)acrylatessuch as n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl(meth)acrylate, n-hexyl (meth)acrylate, octyl (meth)acrylate,2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, tridecyl(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate,isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl(meth)acrylate, tert-butylcyclohexyl (meth)acrylate, cyclododecyl(meth)acrylate and tricyclodecanyl (meth)acrylate; polymerizableunsaturated compounds with isobornyl groups such as isobornyl(meth)acrylate; polymerizable unsaturated compounds with adamantylgroups such as adamantyl (meth)acrylate; aromatic ring-containingpolymerizable unsaturated monomers such as benzyl (meth)acrylate,styrene, α-methylstyrene and vinyltoluene.

[Carboxyl Group-Containing Polymerizable Unsaturated Monomer (m-4)]

Specific examples for the carboxyl group-containing polymerizableunsaturated monomer (m-4) include (meth)acrylic acid, maleic acid,crotonic acid and β-carboxyethyl acrylate.

[Polymerizable Unsaturated Monomer (m-5) Other than (m-1) to (m-4)]

Examples for the polymerizable unsaturated monomer (m-5) other than(m-1) to (m-4) include ≦C3 alkyl (meth)acrylates such as methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate andisopropyl (meth)acrylate; nitrogen-containing polymerizable unsaturatedmonomers containing no urethane bonds, such as (meta)acrylonitrile,(meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate,N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylamide and addition products of glycidyl (meth)acrylate andamines; polymerizable unsaturated monomers with urethane bonds; epoxygroup-containing polymerizable unsaturated monomers such as glycidyl(meth)acrylate, β-methylglycidyl (meth)acrylate,3,4-epoxycyclohexylmethyl (meth)acrylate, 3,4-epoxycyclohexylethyl(meth)acrylate, 3,4-epoxycyclohexylpropyl (meth)acrylate and allylglycidyl ether; (meth)acrylates with polyoxyethylene chains havingalkoxy groups at the molecular ends; polymerizable unsaturated monomerswith sulfonic acid groups, such as 2-acrylamide-2-methylpropanesulfonicacid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid and4-styrenesulfonic acid, as well as sodium salts and ammonium salts ofthese sulfonic acids; polymerizable unsaturated monomers withalkoxysilyl groups, such as vinyltrimethoxysilane, vinyltriethoxysilane,vinyltris(2-methoxyethoxy)silane,γ-(meth)acryloyloxypropyltrimethoxysilane andγ-(meth)acryloyloxypropyltriethoxysilane; perfluoroalkyl (meth)acrylatessuch as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl(meth)acrylate; polymerizable unsaturated monomers with fluorinatedalkyl groups, such as fluoroolefins; and polymerizable unsaturatedmonomers with photopolymerizable functional groups, such as maleimide.

[Core Section Copolymer (I)]

The core section copolymer (I) of the acrylic resin emulsion (a) to beused in the water-based coating composition of the invention comprises apolymerizable unsaturated monomer with two or more polymerizableunsaturated groups in the molecule and a hydroxyl group-containingpolymerizable unsaturated monomer, as copolymerizing components.

The polymerizable unsaturated monomer with two or more polymerizableunsaturated groups in the molecule has the function of imparting acrosslinked structure to the core section copolymer (I). Thepolymerizable unsaturated monomer with two or more polymerizableunsaturated groups in the molecule may be any one or a combination oftwo or more of the polymerizable unsaturated monomers mentioned asexamples for the polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule (m-1), but from theviewpoint of viscosity and film performance of the coating material thatis to be obtained, it is preferred to use methylene bis(meth)acrylamide,allyl (meth)acrylate or ethyleneglycol di(meth)acrylate.

The proportion of the polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule that is used may beappropriately determined according to the degree of crosslinking of thecore section copolymer (I), but for most purposes it is preferably about0.1 to 10 mass %, more preferably about 0.5 to 8 mass % and even morepreferably about 1 to 6 mass %, based on the total mass of monomers thatcan form the core section copolymer (I).

Also, the hydroxyl group-containing polymerizable unsaturated monomerhas the function of improving the chipping resistance and waterresistance of the coating film, by introducing hydroxyl groups thatundergo crosslinking reaction with the curing agent (c) (that isreactive with hydroxyl groups) into the core section copolymer (I). Thehydroxyl group-containing polymerizable unsaturated monomer that is usedmay be a single one or a combination of two or more of the polymerizableunsaturated monomers mentioned as examples for the hydroxylgroup-containing polymerizable unsaturated monomer (m-2), but preferably2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,3-hydroxypropyl (meth)acrylate or 4-hydroxybutyl (meth)acrylate is used,and more preferably 2-hydroxyethyl (meth)acrylate is used.

The hydroxyl value of the core section copolymer (I) is preferably about50 to 200 mgKOH/g, more preferably about 50 to 150 mgKOH/g and even morepreferably about 50 to 100 mgKOH/g, from the viewpoint of stability ofthe acrylic resin emulsion (a) in the aqueous medium and excellent waterresistance of the coating film that is to be obtained.

The core section copolymer (I) used may be a polymerizable unsaturatedmonomer with a C4 or greater hydrocarbon group (m-3) if necessary, inaddition to the aforementioned polymerizable unsaturated monomer withtwo or more polymerizable unsaturated groups in the molecule (m-1) andthe aforementioned hydroxyl group-containing polymerizable unsaturatedmonomer (m-2) from the viewpoint of improving the smoothness anddistinctness of image of the coating film that is to be obtained. Thepolymerizable unsaturated monomer with a C4 or greater hydrocarbon group(m-3) used may be one or a combination of two or more of the monomersmentioned above, for example, and n-butyl (meth)acrylate, isobutyl(meth)acrylate, 2-ethylhexyl (meth)acrylate or styrene is preferablyused.

By adding an acrylic resin emulsion (a) copolymerized with thepolymerizable unsaturated monomer with a C4 or greater hydrocarbon group(m-3) to the water-based first pigmented coating material (A), it ispossible to obtain a first pigmented coating film with lower polarity,and to obtain a multilayer coating film having minimal layer mixing whenthe water-based second pigmented coating material (B) has been appliedonto the uncured first pigmented coating film, and satisfactorysmoothness and distinctness of image.

When the core section includes a copolymerizable unsaturated monomerwith a C4 or greater hydrocarbon group as a copolymerizing component,the proportion of the polymerizable unsaturated monomer with a C4 orgreater hydrocarbon group (m-3) that is used is preferably 30 to 90 mass%, more preferably about 35 to 85 mass % and even more preferably about40 to 80 mass %, based on the total mass of the monomer composing thecore section copolymer (I), from the viewpoint of smoothness anddistinctness of image of the multilayer coating film that is to beobtained.

The polymerizable unsaturated monomer to be used as a monomer for thecore section copolymer (I) is not particularly restricted, other thanthat the polymerizable unsaturated monomers (m-1) and (m-2) areessential components, and for example, any of those mentioned for thepolymerizable unsaturated monomer (m) composing the acrylic resinemulsion (a) may be suitably used, as necessary. In addition to thepolymerizable unsaturated monomer with two or more polymerizableunsaturated groups in the molecule (m-1), the hydroxyl group-containingpolymerizable unsaturated monomer (m-2) and the polymerizableunsaturated monomer with a C4 or greater hydrocarbon group (m-3), otherexamples include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl(meth)acrylate and isopropyl (meth)acrylate. These monomers may be usedalone or in combinations of two or more.

[Shell Section Copolymer (II)]

The shell section copolymer (II) of the acrylic resin emulsion (a) to beused in the water-based coating composition of the invention preferablycomprises a hydroxyl group-containing polymerizable unsaturated monomeras a copolymerizing component.

The hydroxyl group-containing polymerizable unsaturated monomer has thefunction of improving the water resistance of the coating film that isto be formed, and of improving the stability of the acrylic resinemulsion (a) in the aqueous medium, by introducing a hydroxyl group thatundergoes crosslinking reaction with the curing agent (c) (that isreactive with hydroxyl groups) into the acrylic resin emulsion (a).

The hydroxyl group-containing polymerizable unsaturated monomer that isused may be a single one or a combination of two or more of thepolymerizable unsaturated monomers mentioned as examples for thehydroxyl group-containing polymerizable unsaturated monomer (m-2), butpreferably 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 3-hydroxypropyl (meth)acrylate or 4-hydroxybutyl(meth)acrylate is used, and more preferably 2-hydroxyethyl(meth)acrylate is used.

The hydroxyl value of the shell section copolymer (II) is preferablyabout 50 to 200 mgKOH/g, more preferably about 50 to 150 mgKOH/g andeven more preferably about 50 to 100 mgKOH/g, from the viewpoint ofstability of the acrylic resin in the aqueous medium and excellent waterresistance of the coating film that is to be obtained.

The shell section copolymer (II) may also optionally include a carboxylgroup-containing polymerizable unsaturated monomer as a copolymerizingcomponent. The carboxyl group-containing polymerizable unsaturatedmonomer has the function of improving the stability of the acrylic resinemulsion (a) that is to be obtained, in the aqueous medium. The carboxylgroup-containing polymerizable unsaturated monomer used may suitably beone or a combination of two or more of the polymerizable unsaturatedmonomers mentioned above for the carboxyl group-containing polymerizableunsaturated monomer (m-4), but it is preferred to use (meth)acrylicacid.

From the viewpoint of the smoothness and distinctness of image of thecoating film that is to be obtained, the polymerizable unsaturatedmonomer to be used as the monomer for the shell section copolymer (II)is preferably not a polymerizable unsaturated monomer with two or morepolymerizable unsaturated groups in the molecule (m-1), so that thecopolymer (II) is non-crosslinked.

The polymerizable unsaturated monomer to be used as a monomer for theshell section copolymer (II) is not particularly restricted, other thanthat the monomer (m-2) is an essential component, and for example, anyof those mentioned for the polymerizable unsaturated monomer (m)composing the acrylic resin emulsion (a) may be suitably used, asnecessary. In addition to the hydroxyl group-containing polymerizableunsaturated monomer (m-2) and the carboxyl group-containingpolymerizable unsaturated monomer (m-4), examples include methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl(meth)acrylate and n-butyl (meth)acrylate. These monomers may be usedalone or in combinations of two or more.

The core section copolymer (I)/shell section copolymer (II) ratio in theacrylic resin emulsion (a) is preferably about 10/90 to 90/10, morepreferably about 50/50 to 85/15 and even more preferably about 65/35 to80/20 as the solid mass ratio, from the viewpoint of improving thebrightness of the coating film.

From the viewpoint of the storage stability of the coating material andthe smoothness, distinctness of image and water resistance of thecoating film that is to be obtained, the acrylic resin emulsion (a) hasan acid value of preferably about 5 to 25 mgKOH/g, more preferably about8 to 20 mgKOH/g and even more preferably about 10 to 15 mgKOH/g.

The acrylic resin emulsion (a) has a glass transition temperature (Tg)of preferably 20° C. or higher, more preferably about 21° C. to 100° C.and even more preferably about 22° C. to 60° C., for the smoothness anddistinctness of image of the multilayer coating film.

According to the invention, the glass transition temperature Tg is thevalue calculated by the following formula.

1/Tg(K)=W ₁ /T ₁ +W ₂ /T ₂ + . . . W _(n) /T _(n)

Tg(° C.)=Tg(K)−273

In the formula, W₁, W₂ . . . W_(n) represents the mass fraction of eachmonomer, and T₁, T₂ . . . T_(n) represents the glass transitiontemperature Tg(K) of a homopolymer of each monomer.

The glass transition temperature of the homopolymer of each monomer isthe value according to Polymer Handbook Fourth Edition, J. Brandrup, E.H. Immergut, E. A. Grulke, ed. (1999), and the glass transitiontemperatures of monomers not listed in this publication are the valuesmeasured by synthesizing a homopolymer of the monomer to aweight-average molecular weight of about 50,000, and determining theglass transition temperature with a DSC220U by Seiko Instruments, Inc.(differential scanning calorimeter). The measurement was conducted byweighing out 50 mg of sample into a special sample dish and drying it at130° C. for 3 hours, and then raising the temperature from −50° C. to150° C. at a speed of 10° C./min in an inert gas, and reading out thetemperature at the point of inflection of the obtained heat changecurve.

Emulsion of the core section copolymer (I) can be accomplished by amethod known in the prior art, such as emulsion polymerization. Forexample, the emulsion polymerization can be carried out by emulsionpolymerization of the monomer mixture using a polymerization initiatorin the presence of an emulsifying agent.

The emulsifying agent is preferably an anionic emulsifying agent ornonionic emulsifying agent. Examples of anionic emulsifying agentsinclude sodium salts and ammonium salts of alkylsulfonic acids,alkylbenzenesulfonic acids and alkylphosphoric acids. Examples ofnonionic emulsifiers include polyoxyethylene oleyl ether,polyoxyethylene stearyl ether, polyoxyethylene lauryl ether,polyoxyethylene tridecyl ether, polyoxyethylene phenyl ether,polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether,polyoxyethylene monolaurate, polyoxyethylene monostearate,polyoxyethylene monooleate, sorbitan monolaurate, sorbitan monostearate,sorbitan trioleate and polyoxyethylenesorbitan monolaurate.

There may also be used a polyoxyalkylene group-containing anionicemulsifying agent having an anionic group and a polyoxyalkylene groupsuch as polyoxyethylene or polyoxypropylene in the molecule; or areactive anionic emulsifying agent having an anionic group and a radicalpolymerizable unsaturated group in the molecule. Of these it ispreferred to use a reactive anionic emulsifying agent.

The reactive anionic emulsifying agent may be a sodium salt of asulfonic acid compound with a radical polymerizable unsaturated groupsuch as allyl, methallyl, (meth)acryloyl, propenyl or butenyl, or anammonium salt of such a sulfonic acid compound. Of these, ammonium saltsof sulfonic acid compounds with radical polymerizable unsaturated groupsare preferred for excellent water resistance of the coating film that isto be obtained. An example of a commercial product that is an ammoniumsalt of a sulfonic acid compound is “LATEMUL S-180A” (trade name of KaoCorp.).

Among ammonium salts of sulfonic acid compounds with radicalpolymerizable unsaturated groups, there are preferred ammonium salts ofsulfonic acid compounds having radical polymerizable unsaturated groupsand polyoxyalkylene groups. Examples of commercial products that areammonium salts of sulfonic acid compounds with radical polymerizableunsaturated groups and polyoxyalkylene groups include “AQUALON KH-10”(trade name of Dai-ichi Kogyo Seiyaku Co., Ltd.) and “SR-1025A” (tradename of Adeka Corp.).

The amount of the emulsifying agent used is preferably about 0.1 to 15mass %, more preferably about 0.5 to 10 mass % and even more preferablyabout 1 to 5 mass %, based on the total of all of the monomers used.

Examples of polymerization initiators include organic peroxides such asbenzoyl peroxide, octanoyl peroxide, lauroyl peroxide, stearoylperoxide, cumene hydroperoxide, tert-butyl peroxide, tert-butylperoxylaurate, tert-butylperoxyisopropyl carbonate, tert-butylperoxyacetate and diisopropylbenzene hydroperoxide; azo compounds suchas azobisisobutyronitrile, azobis(2,4-dimethylvaleronitrile),azobis(2-methylpropionitrile), azobis(2-methylbutyronitrile),4,4′-azobis(4-cyanobutanoic acid), dimethylazobis(2-methyl propionate),azobis[2-methyl-N-(2-hydroxyethyl)-propionamide] andazobis{2-methyl-N-[2-(1-hydroxybutyl)]-propionamide}; and persulfuricacid salts such as potassium persulfate, ammonium persulfate and sodiumpersulfate. These polymerization initiators may be used either alone orin combinations of two or more. If necessary, a reducing agent such as asugar, sodium formaldehyde sulfoxylate or an iron complex may be usedwith the polymerization initiator, for use as a redox initiator.

Generally, the amount of the polymerization initiator used is preferablyabout 0.1 to 5 mass % and more preferably about 0.2 to 3 mass %, basedon the total mass of all of the monomers used. The method of adding thepolymerization initiator is not particularly restricted, and it may beappropriately selected according to the type and amount used. Forexample, it may be mixed with the monomer mixture or aqueous mediumbeforehand, or it may be added all at once or in a dropwise mannerduring polymerization.

The acrylic resin emulsion (a) can be obtained by adding thepolymerizable unsaturated monomer mixture for the shell section to theobtained emulsion of the core section copolymer (I), and conductingfurther polymerization to form the shell section copolymer (II).

The monomer mixture that is to form the shell section copolymer (II) mayappropriately include components such as polymerization initiators,chain transfer agents, reducing agents, emulsifying agents and the like,as necessary. Also, although the monomer mixture may be added dropwiseas is, preferably it is added dropwise as a monomer emulsion obtained bydispersing the monomer mixture in an aqueous medium. The particlediameter of the monomer emulsion in this case is not particularlyrestricted.

The polymerization method for the monomer mixture that is to form theshell section copolymer (II) may be, for example, a method of adding themonomer mixture or its emulsion to the reaction system either all atonce or in a slow dropwise fashion to the emulsion of the core sectioncopolymer (I), and heating to a suitable temperature while stirring.

The acrylic resin emulsion (a) obtained in this manner may have a meanparticle diameter generally in the range of about 10 to 1,000 nm,preferably about 40 to 500 nm and most preferably about 70 to 200 nm,for water-dispersibility of the core-shell type multilayer structure.

As used herein the mean particle diameter of the acrylic resin emulsion(a) is the value measured using a submicron particle size distributionanalyzer at 20° C., after dilution with deionized water by a commonmethod. As an example of a submicron particle size distributionanalyzer, there may be used a “COULTER N4” (trade name of BeckmanCoulter, Inc.).

When the acrylic resin emulsion (a) has an acidic group such as acarboxyl group, it is preferred to neutralize the acidic group of theacrylic resin emulsion (a) with a neutralizing agent, in order toimprove the mechanical stability of the acrylic resin emulsion (a)particles. There are no particular restrictions on the neutralizingagent so long as it can neutralize the acidic groups, and examplesinclude sodium hydroxide, potassium hydroxide, trimethylamine,2-(dimethylamino)ethanol, 2-amino-2-methyl-1-propanol, triethylamine andammonia water. These neutralizing agents are preferably used in amountssuch that the pH of the aqueous dispersion of acrylic resin emulsion (a)after neutralization is about 6.5 to 9.0.

[Coating Film-Forming Resin (b)]

The coating film-forming resin (b) to be used in the water-based coatingcomposition of the invention is a resin other than the acrylic resinemulsion (a), and examples include polyester resins, water-solubleacrylic resins, urethane resins, polyether resins and polycarbonateresins having one or more hydrophilic groups such as hydroxyl and/orcarboxyl in the molecule. Of these, there is preferred a polyester resin(b1) a water-soluble acrylic resin (b2) and a urethane resin (b3), thereis more preferred a hydroxyl group-containing polyester resin (b1), ahydroxyl group-containing water-soluble acrylic resin (b2), and ahydroxyl group-containing urethane resin (b3), which contain hydroxylgroups, and there is particularly preferred a hydroxyl group-containingpolyester resin (b1) and a hydroxyl group-containing water-solubleacrylic resin (b2). Any of these may be used alone or in combinations oftwo or more.

[Hydroxyl Group-Containing Polyester Resin (b1)]

The hydroxyl group-containing polyester resin (b1) can be produced, forexample, by esterification reaction or transesterification reactionbetween a polybasic acid component and a polyhydric alcohol component.Specifically, the hydroxyl group-containing polyester resin (b1) can beproduced, for example, by conducting esterification reaction with anequivalent ratio of carboxyl groups in the polybasic acid component andhydroxyl groups in the polyhydric alcohol component (COOH/OH) of lessthan 1, so that there are more hydroxyl groups than carboxyl groups.

Also, the acid value of the hydroxyl group-containing polyester resin(b1) can be adjusted, for example, by conducting the esterificationreaction while monitoring the acid value, and completing theesterification reaction when the target acid value has been reached.

The polybasic acid component is a compound having two or more carboxylgroups in the molecule, and examples include polybasic acids such asphthalic acid, isophthalic acid, terephthalic acid, succinic acid,glutaric acid, adipic acid, azelaic acid, sebacic acid,tetrahydrophthalic acid, hexahydrophthalic acid, maleic acid, fumaricacid, itaconic acid, trimellitic acid or pyromellitic acid; anhydridesof these polybasic acids; and lower alkyl esters of these polybasicacids, any one or mixture of two or more of which may be used.

Also, the polyhydric alcohol component is a compound having two or morehydroxyl groups in the molecule, and examples include α-glycols such asethylene glycol, 1,2-propylene glycol, 1,2-butylene glycol, 2,3-butyleneglycol, 1,2-hexanediol, 1,2-dihydroxycyclohexane,3-ethoxypropane-1,2-diol and 3-phenoxypropane-1,2-diol; and neopentylglycol, 2-methyl-1,3-propanediol, 2-methyl-2,4-pentanediol,3-methyl-1,3-butanediol, 2-ethyl-1,3-hexanediol,2,2-diethyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol,2-butyl-2-ethyl-1,3-propanediol, 2-phenoxypropane-1,3-diol,2-methyl-2-phenylpropane-1,3-diol, 1,3-propylene glycol, 1,3-butyleneglycol, 2-ethyl-1,3-octanediol, 1,3-dihydroxycyclohexane,1,4-butanediol, 1,4-dihydroxycyclohexane, 1,5-pentanediol,1,6-hexanediol, 2,5-hexanediol, 3-methyl-1,5-pentanediol,1,4-dimethylolcyclohexane, tricyclodecanedimethanol,2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate (an estercompound of hydroxypivalic acid and neopentyl glycol), bisphenol A,bisphenol F, bis(4-hydroxyhexyl)-2,2-propane,bis(4-hydroxyhexyl)methane,3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane,diethylene glycol, triethylene glycol, glycerin, diglycerin,triglycerin, pentaerythritol, dipentaerythritol, sorbitol, mannitol,trimethylolethane, trimethylolpropane, ditrimethylolpropane andtris(2-hydroxyethyl)isocyanurate. These may be used either alone or incombinations of two or more.

The esterification or transesterification reaction between the polybasicacid component and the polyhydric alcohol component may be carried outby a known method, such as by polycondensation of the polybasic acidcomponent and the polyhydric alcohol component at a temperature of about180° C. to 250° C.

Also, the hydroxyl group-containing polyester resin (b1) can be modifiedwith a fatty acid, monoepoxy compound or the like either duringpreparation of the polyester resin or after the esterification reaction.Examples for the fatty acid include coconut fatty acid, cottonseed oilfatty acid, hempseed oil fatty acid, rice bran oil fatty acid, fish oilfatty acid, tall oil fatty acid, soybean oil fatty acid, linseed oilfatty acid, China wood oil fatty acid, rapeseed oil fatty acid, castoroil fatty acid, dehydrated castor oil fatty acid and safflower oil fattyacid, and a preferred example for the monoepoxy compound is “CARDURAE10P” (trade name of Japan Epoxy Resins Co., Ltd., glycidyl ester ofsynthetic highly-branched saturated fatty acid).

From the viewpoint of the smoothness of the coating film that is to beobtained, the hydroxyl group-containing polyester resin (b1) has an acidvalue of no greater than 50 mgKOH/g, preferably 5 to 40 mgKOH/g and morepreferably 10 to 35 mgKOH/g, and a number-average molecular weight inthe range of 500 to 5,000, preferably 700 to 3,000 and more preferably1,000 to 2,000. From the viewpoint of curability, the hydroxyl value isin the range of 10 to 200 mgKOH/g, preferably 30 to 180 mgKOH/g and morepreferably 50 to 165 mgKOH/g.

The number-average molecular weight or weight-average molecular weight,for the purpose of the present specification, is the value of thenumber-average molecular weight or weight-average molecular weightmeasured with a gel permeation chromatograph (“HLC8120GPC” by TosohCorp.), converted based on the number-average molecular weight orweight-average molecular weight of polystyrene. Four columns were usedfor measurement, namely “TSKgel G-4000H×L”, “TSKgel G-3000H×L”, “TSKgelG-2500H×L” and “TSKgel G-2000H×L” (all trade names of Tosoh Corp.), withthe following conditions: mobile phase: tetrahydrofuran, measuringtemperature: 40° C., flow rate: 1 mL/min, detector: RI.

[Hydroxyl Group-Containing Water-Soluble Acrylic Resin (b2)]

The hydroxyl group-containing water-soluble acrylic resin (b2) can beproduced by (co)polymerization of a polymerizable unsaturated monomercomponent including a hydroxyl group-containing polymerizableunsaturated monomer under ordinary conditions, and from the viewpoint ofthe smoothness of the coating film that is to be obtained, the acidvalue is no greater than 60 mgKOH/g, preferably 1 to 50 mgKOH/g and morepreferably 5 to 40 mgKOH/g, and the weight-average molecular weight isin the range of 1,000 to 100,000, preferably 3,000 to 80,000 and morepreferably 5,000 to 60,000. Also, from the viewpoint of curability, thehydroxyl value is in the range of 20 to 200 mgKOH/g, preferably 30 to180 mgKOH/g and more preferably 50 to 165 mgKOH/g.

The hydroxyl group-containing water-soluble acrylic resin (b2) comprisesa polymerizable unsaturated monomer having a hydrophilic functionalgroup such as an N-substituted (meth)acrylamide, polyoxyalkylenechain-containing (meth)acrylate, hydroxyl group-containing(meth)acrylate and/or acid group-containing (meth)acrylate, and isdissolved (transparent) in the aqueous medium, and therefore is clearlydistinguished from the acrylic resin emulsion (a) which is in adispersed state in the aqueous medium.

The hydroxyl group-containing polymerizable unsaturated monomer is acompound having one or more hydroxyl groups and one or morepolymerizable unsaturated bonds in the molecule, and examples thereofinclude monoesterified products of (meth)acrylic acid and C2-8 dihydricalcohols, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 3-hydroxypropyl (meth)acrylate and 4-hydroxybutyl(meth)acrylate, ε-caprolactone-modified forms of the monoesterifiedproducts of (meth)acrylic acid and C2-8 dihydric alcohols, allylalcohols, and (meth)acrylates having polyoxyethylene chains withmolecular ends that are hydroxyl groups. These may be used either aloneor in combinations of two or more.

Also, examples of other polymerizable unsaturated monomers that arecopolymerizable with the hydroxyl group-containing polymerizableunsaturated monomer include alkyl or cycloalkyl (meth)acrylates such asmethyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate,isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl(meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, octyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate,tridecyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate,“isostearyl acrylate” (trade name of Osaka Organic Chemical Industry,Ltd.), cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate,tert-butylcyclohexyl (meth)acrylate and cyclododecyl (meth)acrylate;polymerizable unsaturated monomers with isobornyl groups, such asisobornyl (meth)acrylate; polymerizable unsaturated monomers withadamantyl groups, such as adamantyl (meth)acrylate; vinyl aromaticcompounds such as styrene, α-methylstyrene and vinyltoluene;polymerizable unsaturated monomers with alkoxysilyl groups, such asvinyltrimethoxysilane, vinyltriethoxysilane,vinyltris(2-methoxyethoxy)silane,γ-(meth)acryloyloxypropyltrimethoxysilane andγ-(meth)acryloyloxypropyltriethoxysilane; perfluoroalkyl (meth)acrylatessuch as perfluorobutylethyl (meth)acrylate and perfluorooctylethyl(meth)acrylate; polymerizable unsaturated monomers with fluorinatedalkyl groups such as fluoroolefins; polymerizable unsaturated monomerswith photopolymerizable functional groups, such as maleimide; vinylcompounds such as N-vinylpyrrolidone, ethylene, butadiene, chloroprene,vinyl propionate and vinyl acetate; carboxyl group-containingpolymerizable unsaturated monomers such as (meth)acrylic acid, maleicacid, crotonic acid and β-carboxyethyl acrylate; nitrogen-containingpolymerizable unsaturated monomers such as (meta)acrylonitrile,(meth)acrylamide, dimethylaminopropyl (meth)acrylamide,dimethylaminoethyl (meth)acrylate and glycidyl (meth)acrylate and amineaddition products; (meth)acrylates with polyoxyethylene chains havingalkoxy groups on the molecular ends; polymerizable unsaturated monomerswith sulfonic acid groups, such as 2-acrylamide-2-methylpropanesulfonicacid, allylsulfonic acid, sodium styrenesulfonate salt, sulfoethylmethacrylate, and their sodium salts or ammonium salts; polymerizableunsaturated monomers with phosphate groups, such as 2-acryloyloxyethylacid phosphate, 2-methacryloyloxyethyl acid phosphate,2-acryloyloxypropyl acid phosphate and 2-methacryloyloxypropyl acidphosphate; polymerizable unsaturated monomers with ultraviolet absorbinggroups, such as addition reaction products of hydroxybenzophenones suchas 2-hydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone,2-hydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone,2,2′-dihydroxy-4-(3-methacryloyloxy-2-hydroxypropoxy)benzophenone and2,2′-dihydroxy-4-(3-acryloyloxy-2-hydroxypropoxy)benzophenone, andglycidyl (meth)acrylate, or2-(2′-hydroxy-5′-methacryloyloxyethylphenyl)-2H-benzotriazole;polymerizable unsaturated monomers with ultraviolet ray stabilizationperformance, such as4-(meth)acryloyloxy-1,2,2,6,6-pentamethylpiperidine,4-(meth)acryloyloxy-2,2,6,6-tetramethylpiperidine,4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine,1-(meth)acryloyl-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine,1-(meth)acryloyl-4-cyano-4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine,4-crotonoyloxy-2,2,6,6-tetramethylpiperidine,4-crotonoylamino-2,2,6,6-tetramethylpiperidine and1-crotonoyl-4-crotonoyloxy-2,2,6,6-tetramethylpiperidine; andpolymerizable unsaturated monomers such as acrolein,diacetoneacrylamide, diacetonemethacrylamide, acetoacetoxyethylmethacrylate, formylstyrol, and vinylalkyl ketones with 4-7 carbon atoms(for example, vinylmethyl ketone, vinylethyl ketone and vinylbutylketone), any of which may be used alone or in combinations of two ormore.

When the hydroxyl group-containing water-soluble acrylic resin (b2) hasan acid value, the acid value can be adjusted, for example, by using acarboxyl group-containing polymerizable unsaturated monomer such as(meth)acrylic acid, maleic acid, crotonic acid or β-carboxyethylacrylate and/or another acid group-containing polymerizable unsaturatedmonomer, as a polymerizable unsaturated monomer component, and adjustingtheir contents.

The acid groups may also be neutralized with a neutralizing agent, ifnecessary. There are no particular restrictions on the neutralizingagent so long as it can neutralize the acidic groups, and examplesinclude sodium hydroxide, potassium hydroxide, trimethylamine,2-(dimethylamino)ethanol, 2-amino-2-methyl-1-propanol, triethylamine andammonia water. These may be used either alone or in combinations of twoor more.

[Curing Agent (c)]

The curing agent (c) is a compound that reacts with the crosslinkablefunctional groups such as hydroxyl, carboxyl or epoxy in the acrylicresin emulsion (a) and/or the coating film-forming resin (b), to allowcuring of the water-based coating composition of the invention. Examplesfor the curing agent (c) include amino resins, polyisocyanate compounds,blocked polyisocyanate compounds, epoxy group-containing compounds,carboxyl group-containing compounds and carbodiimide group-containingcompounds, and of which may be used alone or in combinations of two ormore.

Among these, from the viewpoint of water resistance of the coating filmthat is to be obtained, the curing agent (c) preferably contains anamino resin (c1) and/or blocked polyisocyanate compound (c2) that canreact with hydroxyl groups, and most preferably it contains both anamino resin (c1) and a blocked polyisocyanate compound (c2).

[Amino Resin (c1)]

The amino resin (c1) may be a methylolated amino resin obtained byreaction between an amino component and an aldehyde component, and forexample, partial methylolated amino resins and total methylolated aminoresins may be mentioned. Examples of such amino components includemelamine, urea, benzoguanamine, acetoguanamine, steroguanamine,spiroguanamine and dicyandiamide. Aldehyde components includeformaldehyde, paraformaldehyde, acetaldehyde and benzaldehyde.

The amino resin (c1) used may be one in which the methylol groups of themethylolated amino resin are partially or completely etherified with anappropriate alcohol. Examples of alcohols to be used for theetherification include methyl alcohol, ethyl alcohol, n-propyl alcohol,isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, 2-ethyl-1-butanoland 2-ethyl-1-hexanol.

The amino resin (c1) is most preferably a melamine resin (c1′). Types ofmelamine resins (c1′) include alkyletherified melamine resins,methylolated melamine resins, imino group-containing melamine resins andtheir mixed forms, and in particular there are preferably used methyletherified melamine resins with the methylol groups of a partially ortotally methylolated melamine resin partially or completely etherifiedwith methyl alcohol, butyl etherified melamine resins with the methylolgroups of a partially or totally methylolated melamine resin partiallyor completely etherified with butyl alcohol, methyl etherified melamineresins with the methylol groups of a partially or totally methylolatedmelamine resin partially or completely etherified with methyl alcoholand butyl alcohol, and methyl-butyl mixed etherified melamine resins, toobtain a multilayer coating film with excellent smoothness, distinctnessof image and water resistance, there are more preferably used methyletherified melamine resins. The melamine resin (c1′) used may be asingle one or a combination of two or more different ones.

Also, from the viewpoint of the smoothness and distinctness of image ofthe multilayer coating film that is to be obtained, the melamine resin(c1′) has a weight-average molecular weight of preferably 400 to 3,000,more preferably 450 to 2,000 and even more preferably 500 to 1,000.

The melamine resin (c1′) that is used may be a commercial product.Examples of trade names of commercial products include “SAIMEL 202”,“SAIMEL 203”, “SAIMEL 204”, “SAIMEL 211”, “SAIMEL 238”, “SAIMEL 251”,“SAIMEL 303”, “SAIMEL 323”, “SAIMEL 324”, “SAIMEL 325”, “SAIMEL 327”,“SAIMEL 350”, “SAIMEL 385”, “SAIMEL 1156”, “SAIMEL 1158”, “SAIMEL 1116”,“SAIMEL 1130” (all products of Nihon Cytec Industries Inc.), “U-VAN120”, “U-VAN 20HS”, “U-VAN 20SE60”, “U-VAN 2021”, “U-VAN 2028” and“U-VAN 28-60” (all products of Mitsui Chemicals, Inc.).

Also, when the curing agent (c) employs the aforementioned amino resin(c1), a sulfonic acid such as para-toluenesulfonic acid,dodecylbenzenesulfonic acid or dinonylnaphthalenesulfonic acid or a saltof such a sulfonic acid with an amine compound may be used as acatalyst.

[Blocked Polyisocyanate Compound (c2)]

The blocked polyisocyanate compound (c2) is not particularly restricted,and may be synthesized using a common polyisocyanate compound andblocking agent.

The polyisocyanate compound can be obtained using the followingdiisocyanates as the main starting materials.

Diisocyanates are aliphatic and/or alicyclic diisocyanates. Aliphaticdiisocyanates are preferably C4-30 while alicyclic diisocyanates arepreferably C8-30, and examples include 1,4-tetramethylene diisocyanate,1,5-pentamethylene diisocyanate, 1,6-hexamethylene diisocyanate,2,2,4-trimethyl-1,6-hexamethylene diisocyanate, lysine diisocyanate,isophorone diisocyanate, 1,3-bis(isocyanatomethyl)-cyclohexane and4,4′-dicyclohexylmethane diisocyanate. Preferred among these are1,6-hexamethylene diisocyanate or isophorone diisocyanate, for weatherresistance and/or industrial availability, and they may be used alone orin combinations.

The polyisocyanate compound has about 2 to 20 isocyanate groups in themolecule, and for example, it is a dimer to 20mer oligomer ofdiisocyanate produced by forming biuret bonds, urea bonds, isocyanuratebonds, urethodione bonds, urethane bonds, allophanate bonds,oxadiazinetrione bonds or the like.

The blocking agent used for blocking of the isocyanate groups of thepolyisocyanate compound may be a publicly known one, and for example,there may be suitably used active methylene-based, phenol-based,alcohol-based, mercaptane-based, acid amide-based, imide-based,amine-based, imidazole-based, urea-based, carbamate-based, imine-based,oxime-based and sulfite-based compounds.

Also, the blocked polyisocyanate compound is preferably a blockedpolyisocyanate compound with a hydrophilic group, from the viewpoint ofstorage stability and curability of the water-based coating composition,and smoothness and distinctness of image of the multilayer coating filmthat is to be formed.

The blocked polyisocyanate compound with a hydrophilic group can beobtained, for example, by using an active hydrogen-containing compoundwith a hydrophilic group, such as an active hydrogen-containing compoundwith a nonionic hydrophilic group, an active hydrogen-containingcompound with an anionic hydrophilic group, or an activehydrogen-containing compound with a cationic hydrophilic group.

Preferred for the active hydrogen-containing compound with a hydrophilicgroup are active hydrogen-containing compounds with nonionic hydrophilicgroups, among which polyethyleneglycol monomethyl ether,polyethyleneglycol monoethyl ether and polyethylene glycol arepreferred, and polyethyleneglycol monomethyl ether is more preferred.

Also, an active methylene-type blocked polyisocyanate compound (c3) maybe suitably used as the blocked polyisocyanate compound.

[Active Methylene-Type Blocked Polyisocyanate Compound (c3)]

The blocked polyisocyanate compound (c3) comprises an activemethylene-type blocked polyisocyanate compound with at least one type ofblocked isocyanate group selected from the group consisting of blockedisocyanate groups represented by the following formula (I):

(wherein R¹, R², R⁴ and R⁵ each independently represent a C1-12hydrocarbon group and R³ represents a C1-12 straight-chain or branchedalkylene group), blocked isocyanate groups represented by the followingformula (II):

(wherein R², R³, R⁴ and R⁵ have the same definitions as above),and blocked isocyanate groups represented by the following formula(III):

(wherein R², R³, R⁴ and R⁵ have the same definitions as above, and R⁶represents a C1-12 hydrocarbon group).

The method of synthesizing the active methylene-type blockedpolyisocyanate compound (c3) may be, for example, a method of reactingan active methylene compound (cb2) with the isocyanate groups of apolyisocyanate compound (cb1) with two or more isocyanate groups in themolecule, to obtain an active methylene-type blocked polyisocyanatecompound (cb3), and then reacting the obtained active methylene-typeblocked polyisocyanate compound (cb3) with a secondary alcohol (cb4), ora method of reacting the reaction product of an active methylenecompound (cb2) and a secondary alcohol (cb4) with the isocyanate groupsof a polyisocyanate compound (cb1), with the former method beingespecially preferred.

[Polyisocyanate Compound (cb1)]

The polyisocyanate compound (cb1) is a compound with at least twoisocyanate groups in the molecule, and examples include aliphaticpolyisocyanates, alicyclic polyisocyanates, aromatic/aliphaticpolyisocyanates, aromatic polyisocyanates, and derivatives of thesepolyisocyanates.

Examples of aliphatic polyisocyanates include aliphatic diisocyanatessuch as trimethylene diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylenediisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate,1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylenediisocyanate, dimer acid diisocyanate and methyl2,6-diisocyanatohexanoate (common name: lysine diisocyanate); andaliphatic triisocyanates such as 2-isocyanatoethyl2,6-diisocyanatohexanoate, 1,6-diisocyanato-3-isocyanatomethylhexane,1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane,1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane and2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane.

Examples of alicyclic polyisocyanates include alicyclic diisocyanatessuch as 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate,1,3-cyclohexane diisocyanate,3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name:isophorone diisocyanate), 4-methyl-1,3-cyclohexylene diisocyanate(common name: hydrogenated TDI), 2-methyl-1,3-cyclohexylenediisocyanate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name:hydrogenated xylylene diisocyanate) or mixtures thereof, methylenebis(4,1-cyclohexanediyl)diisocyanate (common name: hydrogenated MDI) andnorbornane diisocyanate; and alicyclic triisocyanates such as1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane,2-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane,2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo(2.2.1)heptane,3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo(2.2.1)heptane,5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane,6-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane,5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)-heptaneand6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo(2.2.1)heptane.

Examples of aromatic/aliphatic polyisocyanates includearomatic/aliphatic diisocyanates such as methylenebis(4,1-phenylene)diisocyanate (common name: MDI), 1,3- or 1,4-xylylenediisocyanate or mixtures thereof, ω,ω′-diisocyanato-1,4-diethylbenzeneand 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name:tetramethylxylylene diisocyanate) or mixtures thereof; andaromatic/aliphatic triisocyanates such as1,3,5-triisocyanatomethylbenzene.

Examples of aromatic polyisocyanates include aromatic diisocyanates suchas m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4′-diphenyldiisocyanate, 1,5-naphthalene diisocyanate, 2,4-tolylene diisocyanate(common name: 2,4-TDI) or 2,6-tolylene diisocyanate (common name:2,6-TDI), or mixtures thereof, 4,4′-toluidine diisocyanate and4,4′-diphenyl ether diisocyanate; aromatic triisocyanates such astriphenylmethane-4,4′,4″-triisocyanate, 1,3,5-triisocyanatobenzene and2,4,6-triisocyanatotoluene; and aromatic tetraisocyanates such as4,4′-diphenylmethane-2,2′,5,5′-tetraisocyanate.

Examples of the aforementioned polyisocyanate derivatives include theaforementioned polyisocyanate dimers, trimers, biurets, allophanates,urethodiones, urethoimines, isocyanurates, oxadiazinetriones and thelike, as well as polymethylenepolyphenyl polyisocyanate (crude MDI,polymeric MDI) and crude TDI.

These polyisocyanates and their derivatives may be used alone or incombinations of two or more. Also, among these polyisocyanates,aliphatic diisocyanates, alicyclic diisocyanates and their derivativesare preferred as the polyisocyanate compound (cb1), because theyminimize yellowing of the obtained active methylene-type blockedpolyisocyanate compound (c3) during heating. From the viewpoint ofimproving flexibility of the coating film that is to be formed,aliphatic diisocyanates and their derivatives are more preferred.

Also, the polyisocyanate compound (cb1) that is used may be a prepolymerobtained by reacting any of the aforementioned polyisocyanates and theirderivatives with a compound that can react with the polyisocyanate,under conditions with an excess of isocyanate groups. Compounds that canreact with the aforementioned polyisocyanates include compounds withactive hydrogen groups such as hydroxyl and amino, and specificallythere may be used polyhydric alcohols, low molecular weight polyesterresins, amines, water and the like.

Also, the polyisocyanate compound (cb1) used may be a polymer of anisocyanate group-containing polymerizable unsaturated monomer, or acopolymer of such an isocyanate group-containing polymerizableunsaturated monomer and a polymerizable unsaturated monomer other thanthe isocyanate group-containing polymerizable unsaturated monomer.

From the viewpoint of reactivity of the active methylene-type blockedpolyisocyanate compound (c3) that is to be obtained and compatibilitybetween the active methylene-type blocked polyisocyanate compound (c3)and the other coating material component, the polyisocyanate compound(cb1) has a number-average molecular weight of preferably in the rangeof 300 to 20,000, more preferably in the range of 400 to 8,000 and evenmore preferably in the range of 500 to 2,000.

Also, from the viewpoint of reactivity of the active methylene-typeblocked polyisocyanate compound (c3) that is to be obtained andcompatibility between the active methylene-type blocked polyisocyanatecompound (c3) and the other coating material component, thepolyisocyanate compound (cb1) has an average number of isocyanatefunctional groups in the molecule in the range of 2 to 100. The lowerlimit is more preferably 3 from the viewpoint of increasing thereactivity of the active methylene-type blocked polyisocyanate compound(c3) that is to be obtained. The upper limit is more preferably 20 fromthe viewpoint of preventing gelling during production of the activemethylene-type blocked polyisocyanate compound (c3).

[Active Methylene Compound (cb2)]

Examples for the active methylene compound (cb2) that blocks theisocyanate groups in the polyisocyanate compound (cb1) include malonicacid diesters such as dimethyl malonate, diethyl malonate, di-n-propylmalonate, diisopropyl malonate, di-n-butyl malonate, diisobutylmalonate, disec-butyl malonate, di-tert-butyl malonate, di-n-pentylmalonate, di-n-hexyl malonate, di(2-ethylhexyl) malonate,methylisopropyl malonate, ethylisopropyl malonate, methyl-n-butylmalonate, ethyl-n-butyl malonate, methylisobutyl malonate, ethylisobutylmalonate, methyl-sec-butyl malonate, ethyl-sec-butyl malonate, diphenylmalonate and dibenzyl malonate, acetoacetic acid esters such as methylacetoacetate, ethyl acetoacetate, n-propyl acetoacetate, isopropylacetoacetate, n-butyl acetoacetate, isobutyl acetoacetate, sec-butylacetoacetate, tert-butyl acetoacetate, n-pentyl acetoacetate, n-hexylacetoacetate, 2-ethylhexyl acetoacetate, phenyl acetoacetate and benzylacetoacetate, and isobutyrylacetic acid esters such as methylisobutyrylacetate, ethyl isobutyrylacetate, n-propyl isobutyrylacetate,isopropyl isobutyrylacetate, n-butyl isobutyrylacetate, isobutylisobutyrylacetate, sec-butyl isobutyrylacetate, tert-butylisobutyrylacetate, n-pentyl isobutyrylacetate, n-hexylisobutyrylacetate, 2-ethylhexyl isobutyrylacetate, phenylisobutyrylacetate and benzyl isobutyrylacetate, any of which may be usedalone or in combinations of two or more.

From the viewpoint of the smoothness and distinctness of image of themultilayer coating film that is to be formed, the active methylenecompound (cb2) is preferably at least one type of compound selected fromthe group consisting of dimethyl malonate, diethyl malonate, diisopropylmalonate, methyl acetoacetate, ethyl acetoacetate, methylisobutyrylacetate and ethyl isobutyrylacetate, and more preferably atleast one type of compound selected from the group consisting ofdiisopropyl malonate, methyl isobutyrylacetate and ethylisobutyrylacetate. Diisopropyl malonate is more preferred among thesefrom the viewpoint of the smoothness and distinctness of image of themultilayer coating film that is to be formed, the reactivity of theactive methylene-type blocked polyisocyanate compound (c3) that is to beobtained and the storage stability of the water-based coatingcomposition.

The blocking reaction of isocyanate groups by the active methylenecompound (cb2) may employ a reaction catalyst if necessary. Acceptableexamples for the reaction catalyst include basic compounds such as metalhydroxides, metal alkoxides, metal carboxylates, metal acetylacetonates,onium salt hydroxides, onium carboxylates, metal salts of activemethylene compounds, onium salts of active methylene compounds,aminosilanes, amines, phosphines, and the like. Preferred among these asonium salts are ammonium salts, phosphonium salts and sulfonium salts.The amount of reaction catalyst used is, for most cases, preferably inthe range of 10 to 10,000 ppm and more preferably in the range of 20 to5,000 ppm, based on the total solid mass of the polyisocyanate compound(cb1) and the active methylene compound (cb2).

The blocking reaction of isocyanate groups by the active methylenecompound (cb2) may be conducted at 0° C. to 150° C. using a solvent. Inthis case, the solvent is preferably an aprotic solvent, with solventssuch as esters, ethers, N-alkylamides and ketones being especiallypreferred. If the reaction has proceeded as designed, an acid componentmay be added to neutralize the basic compound used as the catalyst, andhalt the reaction.

There are no particular restrictions on the amount of active methylenecompound (cb2) used for the blocking reaction of the isocyanate groupsby the active methylene compound (cb2), but 0.1 to 3 mol and preferably0.2 to 2 mol may be used with respect to 1 mol of isocyanate groups inthe polyisocyanate compound (cb1). Also, the active methylene compoundthat has not reacted with isocyanate groups in the polyisocyanatecompound (cb1) may be removed after completion of the blocking reaction.

Also, together with the active methylene compound (cb2) there may beadded, for example, an alcohol-based, phenol-based, oxime-based,amine-based, acid amide-based, imidazole-based, pyridine-based ormercaptane-based blocking agent.

Some of the isocyanate groups in the polyisocyanate compound (cb1) mayalso be reacted with an active hydrogen-containing compound. By reactingsome of the isocyanate groups in the polyisocyanate compound (cb1) withan active hydrogen-containing compound, it is possible to improve thestorage stability of the active methylene-type blocked polyisocyanatecompound (c3) that is obtained, to modify the compatibility between theactive methylene-type blocked polyisocyanate compound (c3) and the othercoating material components, and to increase flexibility of the coatingfilm that is to be formed.

When some of the isocyanate groups in the polyisocyanate compound (cb1)are to be reacted with the active hydrogen-containing compound, theorder of reacting the polyisocyanate compound (cb1), the activemethylene compound (cb2) and the active hydrogen-containing compound isnot particularly restricted. Specifically, there may be mentioned amethod of blocking some of the isocyanate groups in a polyisocyanatecompound (cb1) with an active methylene compound (cb2), and thenreacting an active hydrogen-containing compound with the remainingisocyanate groups, a method of reacting an active hydrogen-containingcompound with some of the isocyanate groups in a polyisocyanate compound(cb1) and then blocking the remaining isocyanate groups with an activemethylene compound (cb2), and a method of simultaneously reacting anactive methylene compound (cb2) and an active hydrogen-containingcompound with the isocyanate groups in a polyisocyanate compound (cb1).

Examples for the active hydrogen-containing compound include hydroxylgroup-containing compounds and amino group-containing compounds.

Examples of hydroxyl group-containing compounds include propanol,butanol, pentanol, hexanol, heptanol, 2-ethyl-1-hexanol, octanol,nonanol, decanol, tridecanol, stearyl alcohol, ethylene glycol,propylene glycol, polyethylene glycol, polypropylene glycol,polyethylene glycol (propylene glycol), polyethyleneglycol monoalkylethers, polypropyleneglycol monoalkyl ethers, polyethylene glycol(propylene glycol), monoalkyl ethers and trimethylolpropane, any ofwhich may be used alone or in combinations of two or more. As usedherein, “polyethylene glycol (propylene glycol)” means a copolymer ofethylene glycol and propylene glycol, and it includes both their blockcopolymers and random copolymers.

The hydroxyl group-containing compound is preferably a monovalentalcohol from the viewpoint of minimizing viscosity increase of theactive methylene-type blocked polyisocyanate compound (c3) that is to beobtained. Examples of monovalent alcohols include propanol, butanol,pentanol, hexanol, heptanol, 2-ethyl-1-hexanol, octanol, nonanol,decanol, tridecanol, stearyl alcohol, polyethyleneglycol monoalkylether, polypropyleneglycol monoalkyl ether, polyethylene glycol(propylene glycol) and monoalkyl ethers, any of which may be used aloneor in combinations of two or more.

Also, examples for the amino group-containing compound includebutylamine, octylamine, stearylamine, dibutylamine, dioctylamine,dicyclohexylamine, dilaurylamine, α-(aminoalkyl)-ω-alkoxypolyoxyethylene(oxypropylene)s, hexamethylenediamine, diethylenetriamine andpolyoxypropylene-α,ω-diamine (examples of commercial products including“JEFFAMINE D-400” by Huntsman Corp.), any of which may be used alone orin combinations of two or more.

The amino group-containing compound is preferably a monovalent aminefrom the viewpoint of minimizing viscosity increase of the activemethylene-type blocked polyisocyanate compound (c3) that is to beobtained. Examples of monovalent amines include butylamine, octylamine,stearylamine, dibutylamine, dioctylamine, dicyclohexylamine,dilaurylamine and α-(aminoalkyl)-ω-alkoxypolyoxyethylene(oxypropylene)s, any of which may be used alone or in combinations oftwo or more.

When the active hydrogen-containing compound is to be reacted with someof the isocyanate groups in the polyisocyanate compound (cb1), thereacting proportion of the polyisocyanate compound (cb1) and the activehydrogen-containing compound is preferably such that the number of molesof active hydrogen in the active hydrogen-containing compound is in therange of 0.03 to 0.6 mol based on 1 mol of isocyanate groups in thepolyisocyanate compound (cb1), from the viewpoint of storage stabilityand curability of the water-based coating composition, and smoothness,distinctness of image and water resistance of the multilayer coatingfilm that is to be formed. The upper limit is preferably 0.4 and morepreferably 0.3, from the viewpoint of curability of the water-basedcoating composition and water resistance of the multilayer coating filmthat is to be formed. The lower limit is preferably 0.04 and morepreferably 0.05, from the viewpoint of storage stability of thewater-based coating composition and smoothness and distinctness of imageof the multilayer coating film that is to be formed.

Also, the active methylene-type blocked polyisocyanate compound (c3) ispreferably an active methylene-type blocked polyisocyanate compound(c3′) with a hydrophilic group, from the viewpoint of storage stabilityand curability of the water-based coating composition, and smoothnessand distinctness of image of the multilayer coating film that is to beformed.

The active methylene-type blocked polyisocyanate compound (c3′) with ahydrophilic group may be obtained, for example, using an activehydrogen-containing compound with a hydrophilic group as the activehydrogen-containing compound.

The active hydrogen-containing compound with a hydrophilic group may bean active hydrogen-containing compound with a nonionic hydrophilicgroup, an active hydrogen-containing compound with an anionichydrophilic group, an active hydrogen-containing compound with acationic hydrophilic group, or the like, and any one or a combination oftwo or more of these may be used. Of these, it is preferred to use anactive hydrogen-containing compound with a nonionic hydrophilic group,in order to minimize inhibition of the reaction of blocking theisocyanate groups in the polyisocyanate compound (cb1) with the activemethylene compound (cb2).

Examples of active hydrogen-containing compounds having nonionichydrophilic groups that are suitable for use include activehydrogen-containing compounds having polyoxyalkylene groups. Examples ofpolyoxyalkylene groups include polyoxyethylene, polyoxypropylene andpolyoxyethylene (oxypropylene), any of which may be used alone or incombinations of two or more. Of these, active hydrogen-containingcompounds with polyoxyethylene groups are preferred from the viewpointof storage stability of the water-based coating composition.

The active hydrogen-containing compound with a polyoxyethylene group mayhave 3 or more, preferably 5 to 100 and more preferably 8 to 45 linkedoxyethylene groups, from the viewpoint of storage stability of thewater-based coating composition and water resistance of the multilayercoating film that is to be formed.

Also, the active hydrogen-containing compound with a polyoxyethylenegroup may contain an oxyalkylene group other than an oxyethylene group,in addition to the linked oxyethylene groups. Examples of oxyalkylenegroups other than the oxyethylene groups include oxypropylene,oxybutylene and oxyhexylene. The molar ratio of oxyethylene groups amongthe oxyalkylene groups in the active hydrogen-containing compound withpolyoxyethylene groups is preferably in the range of 20 to 100 mol % andmore preferably in the range of 50 to 100 mol %, from the viewpoint ofstorage stability of the water-based coating composition. If the molarratio of oxyethylene groups among the oxyalkylene groups is less than 20mol %, the hydrophilicity imparted may be inadequate, and the storagestability of the water-based coating composition.

Also, the active hydrogen-containing compound with a nonionichydrophilic group preferably has a number-average molecular weight inthe range of 200 to 2,000, from the viewpoint of the storage stabilityof the water-based coating composition and the water resistance of themultilayer coating film that is to be formed. The lower limit for thenumber-average molecular weight is more preferably 300 and even morepreferably 400 from the viewpoint of the storage stability of thewater-based coating composition. The upper limit is preferably 1,500 andmore preferably 1,200, from the viewpoint of the water resistance of themultilayer coating film that is to be formed.

Examples of active hydrogen-containing compounds with nonionichydrophilic groups include polyethyleneglycol monoalkyl ethers(alternate name: ω-alkoxypolyoxyethylenes) such as polyethyleneglycolmonomethyl ether and polyethyleneglycol monoethyl ether,polypropyleneglycol monoalkyl ethers (alternate name:ω-alkoxypolyoxypropylenes) such as polypropyleneglycol monomethyl etherand polypropyleneglycol monoethyl ether,ω-alkoxypolyoxyethylene(oxypropylene)s such asω-methoxypolyoxyethylene(oxypropylene) andω-ethoxypolyoxyethylene(oxypropylene), polyethylene glycol (propyleneglycol)monoalkyl ethers such as polyethylene glycol (propyleneglycol)monomethyl ether and polyethylene glycol (propyleneglycol)monoethyl ether, and polyethylene glycol, polypropylene glycol,polyethylene glycol(propylene glycol),α-(aminoalkyl)-ω-alkoxypolyoxyethylene,α-(aminoalkyl)-ω-alkoxypolyoxypropylene,α-(aminoalkyl)-ω-alkoxypolyoxyethylene and the like, any of which may beused alone or in combinations of two or more. Preferred among these arepolyethyleneglycol monomethyl ether, polyethyleneglycol monoethyl etherand polyethylene glycol, with polyethyleneglycol monomethyl ether beingmore preferred.

Examples of commercial products of polyethyleneglycol monomethyl etherinclude “UNIOX M-400”, “UNIOX M-550”, “UNIOX M-1000” and “UNIOX M-2000”by NOF Corp. Also, examples of commercial products of polyethyleneglycol include “PEG#200”, “PEG#300”, “PEG#400”, “PEG#600”, “PEG#1000”,“PEG#1500”, “PEG#1540” and “PEG#2000” by NOF Corp.

Examples of active hydrogen-containing compounds with anionichydrophilic groups include active hydrogen-containing compounds withcarboxyl groups, active hydrogen-containing compounds with sulfonic acidgroups and active hydrogen-containing compounds with phosphate groups,as well as their neutralized salts, and any of these may be used aloneor in combinations of two or more. Of these, active hydrogen-containingcompounds with carboxyl groups are preferred for use from the viewpointof compatibility between the active methylene-type blockedpolyisocyanate compound (c3) to be obtained and the other coatingmaterial components.

Some or all of the acidic groups in the active hydrogen-containingcompound with an anionic hydrophilic group are preferably neutralizedwith a basic compound, in order to inhibit blocking reaction of theisocyanate groups in the polyisocyanate compound (cb1) by the activemethylene compound (cb2).

Neutralization of the acidic groups in the active hydrogen-containingcompound with an anionic group may be accomplished before the reactionbetween the active hydrogen-containing compound with an anionic groupand the polyisocyanate compound (cb1), or after the reaction.

Examples of basic compounds include hydroxides of alkali metals oralkaline earth metals such as sodium hydroxide, potassium hydroxide,lithium hydroxide, calcium hydroxide and barium hydroxide; metalalkoxides; ammonia; primary monoamines such as ethylamine, propylamine,butylamine, benzylamine, monoethanolamine,2,2-dimethyl-3-amino-1-propanol, 2-aminopropanol,2-amino-2-methyl-1-propanol and 3-aminopropanol; secondary monoaminessuch as diethylamine, diethanolamine, di-n-propanolamine,di-isopropanolamine, N-methylethanolamine and N-ethylethanolamine;tertiary monoamines such as dimethylethanolamine, trimethylamine,triethylamine, triisopropylamine, methyldiethanolamine and2-(dimethylamino)ethanol; and polyamines such as diethylenetriamine,hydroxyethylaminoethylamine, ethylaminoethylamine andmethylaminopropylamine, any of which may be used alone or incombinations of two or more. The amount of basic compound used maygenerally be in the range of 0.1 to 1.5 equivalents and preferably 0.2to 1.2 equivalents, with respect to the anionic groups in the activehydrogen-containing compound with an anionic group.

Examples of active hydrogen-containing compounds having carboxyl groupsinclude monohydroxycarboxylic acids such as glycolic acid, lactic acid,hydroxypivalic acid, malic acid and citric acid, dihydroxycarboxylicacids such as 2,2-dimethylolacetic acid, 2,2-dimethylollactic acid,2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid,dimethylolheptanoic acid, dimethylolnonanoic acid, 2,2-dimethylolbutyricacid and 2,2-dimethylolvaleric acid, lactone ring-opening additionproducts of these dihydroxycarboxylic acids, glycine,1-carboxy-1,5-pentylenediamine, dihydroxybenzoic acid,3,5-diaminobenzoic acid, lysine, arginine, and the like.

Examples of active hydrogen-containing compounds having sulfonic acidgroups include 2-amino-1-ethanesulfonic acid,N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid,1,3-phenylenediamine-4,6-disulfonic acid, diaminobutanesulfonic acid,3,6-diamino-2-toluenesulfonic acid, 2,4-diamino-5-toluenesulfonic acid,2-(cyclohexylamino)-ethanesulfonic acid and3-(cyclohexylamino)-propanesulfonic acid.

Examples of active hydrogen-containing compounds having phosphate groupsinclude 2,3-dihydroxypropylphenyl phosphate, hydroxyalkylphosphonicacids and aminoalkylphosphonic acids.

When the active hydrogen-containing compound with a hydrophilic group isto be reacted with some of the isocyanate groups in the polyisocyanatecompound (cb1), the reacting proportion of the polyisocyanate compound(cb1) and the active hydrogen-containing compound with a hydrophilicgroup is preferably such that the number of moles of active hydrogen inthe active hydrogen-containing compound is in the range of 0.03 to 0.6mol based on 1 mol of isocyanate groups in the polyisocyanate compound(cb1), from the viewpoint of storage stability and curability of thewater-based coating composition, and smoothness, distinctness of imageand water resistance of the multilayer coating film that is to beformed. The upper limit is preferably 0.4 and more preferably 0.3, fromthe viewpoint of curability of the water-based coating composition andwater resistance of the multilayer coating film that is to be formed.The lower limit is preferably 0.04 and more preferably 0.05, from theviewpoint of storage stability of the water-based coating compositionand smoothness and distinctness of image of the multilayer coating filmthat is to be formed.

The active methylene-type blocked polyisocyanate compound (c3) can beimparted with water dispersibility by premixture with a surfactant. Inthis case, the surfactant is preferably a nonionic surfactant and/oranionic surfactant, from the viewpoint of stability of the coatingmaterial.

[Active Methylene-Type Blocked Polyisocyanate Compound (cb3)]

The active methylene-type blocked polyisocyanate compound (cb3) is acompound in which some or all of the isocyanate groups in apolyisocyanate compound (cb1) are blocked with an active methylenecompound (cb2) by reacting the polyisocyanate compound (cb1) having twoor more isocyanate groups in the molecule with the active methylenecompound (cb2).

Among these, it is preferred for the active methylene-type blockedpolyisocyanate compound (cb3) to be either or both an activemethylene-type blocked polyisocyanate compound (cb3-1) with a blockedisocyanate group, represented by the following formula (IV):

(wherein each R¹ independently represents a C1-12 hydrocarbon group, andmay be the same or different), or an active methylene-type blockedpolyisocyanate compound (cb3-2) with a blocked isocyanate group,represented by the following formula (V):

(wherein R⁶ and R⁷ each independently represent a C1-12 hydrocarbongroup).[Active Methylene-Type Blocked Polyisocyanate Compound (cb3-1)]

The active methylene-type blocked polyisocyanate compound (cb3-1) is anactive methylene-type blocked polyisocyanate compound with a blockedisocyanate group, represented by formula (IV) above.

Among these, preferably R¹ is a C1-3 alkyl group in the active methylenecompound (cb2), as one of the starting materials for the activemethylene-type blocked polyisocyanate compound, from the viewpoint ofallowing use of an active methylene compound that can be producedrelatively easily. Of these, from the viewpoint of improvingcompatibility between the active methylene-type blocked polyisocyanatecompound (c3) that is to be obtained and the other coating materialcomponents, R¹ is more preferably a C2 or C3 alkyl group, and from theviewpoint of storage stability of the water-based coating compositionand the smoothness and distinctness of image of the multilayer coatingfilm that is to be formed, R¹ is more preferably an isopropyl group.

The active methylene-type blocked polyisocyanate compound (cb3-1) can beobtained, for example, by reacting the polyisocyanate compound (cb1)with a dialkyl malonate having a C1-12 hydrocarbon group.

Examples of such dialkyl malonates include dimethyl malonate, diethylmalonate, di-n-propyl malonate, diisopropyl malonate, di-n-butylmalonate, diisobutyl malonate, disec-butyl malonate, di-tert-butylmalonate, di-n-pentyl malonate, di-n-hexyl malonate and di(2-ethylhexyl)malonate, any of which may be used alone or in combinations of two ormore. Of these, dimethyl malonate, diethyl malonate, di-n-propylmalonate, diisopropyl malonate, di-n-butyl malonate, diisobutylmalonate, di-sec-butyl malonate and di-tert-butyl malonate arepreferred, diethyl malonate, di-n-propyl malonate and diisopropylmalonate are more preferred, and diisopropyl malonate is even morepreferred.

[Active Methylene-Type Blocked Polyisocyanate Compound (cb3-2)]

The active methylene-type blocked polyisocyanate compound (cb3-2) is anactive methylene-type blocked polyisocyanate compound with a blockedisocyanate group, represented by formula (V) above.

Among these, preferably R⁶ and R⁷ are C1-3 alkyl groups in the activemethylene compound (cb2), as one of the starting materials for theactive methylene-type blocked polyisocyanate compound, from theviewpoint of allowing use of an active methylene compound that can beproduced relatively easily. From the viewpoint of improvingcompatibility between the active methylene-type blocked polyisocyanatecompound (c3) that is to be obtained and the other coating materialcomponents, R⁶ and R⁷ are more preferably C2 or C3 alkyl groups, andfrom the viewpoint of storage stability of the water-based coatingcomposition, and the smoothness and distinctness of image of themultilayer coating film that is to be formed, R⁶ and R⁷ are even morepreferably isopropyl groups.

The active methylene-type blocked polyisocyanate compound (cb3-2) can beobtained, for example, by reacting the polyisocyanate compound (cb1) andan acetoacetic acid ester having a C1-12 hydrocarbon group, or byreacting the polyisocyanate compound (cb1) with an isobutyrylacetic acidester having a C1-12 hydrocarbon group. Of these methods, it ispreferably obtained by reacting the polyisocyanate compound (cb1) withan isobutyrylacetic acid ester having a C1-12 hydrocarbon group.

Examples for the isobutyrylacetic acid ester include methylisobutyrylacetate, ethyl isobutyrylacetate, n-propyl isobutyrylacetate,isopropyl isobutyrylacetate, n-butyl isobutyrylacetate, isobutylisobutyrylacetate, sec-butyl isobutyrylacetate, tert-butylisobutyrylacetate, n-pentyl isobutyrylacetate, n-hexylisobutyrylacetate, 2-ethylhexyl isobutyrylacetate, phenylisobutyrylacetate and benzyl isobutyrylacetate, any of which may be usedalone or in combinations of two or more. Preferred among these aremethyl isobutyrylacetate, ethyl isobutyrylacetate and isopropylisobutyrylacetate.

Also, examples of acetoacetic acid esters include methyl acetoacetate,ethyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate,n-butyl acetoacetate, isobutyl acetoacetate, sec-butyl acetoacetate,tert-butyl acetoacetate, n-pentyl acetoacetate, n-hexyl acetoacetate,2-ethylhexyl acetoacetate, phenyl acetoacetate and benzyl acetoacetate,any of which may be used alone or in combinations of two or more.Preferred among these are methyl acetoacetate, ethyl acetoacetate andisopropyl acetoacetate.

Also, the active methylene-type blocked polyisocyanate compound (cb3)may be a compound obtained by reacting a polyisocyanate compound (cb1)having two or more isocyanate groups in the molecule, an activemethylene compound (cb2) and the aforementioned activehydrogen-containing compound. Specifically, by using the aforementionedactive hydrogen-containing compound with a polyoxyalkylene group, forexample, as the aforementioned active hydrogen-containing compound, itis possible to produce an active methylene-type blocked polyisocyanatecompound wherein some of the isocyanate groups in the polyisocyanatecompound (cb1) are blocked with an active methylene compound (cb2), andsome or all of the other isocyanate groups have reacted with the activehydrogen-containing compound with a polyoxyalkylene group.

[Secondary Alcohol (cb4)]

According to the invention, the active methylene-type blockedpolyisocyanate compound (c3) can be obtained, for example, by reactingthe active methylene-type blocked polyisocyanate compound (cb3) with assecondary alcohol (cb4) represented by the following formula (VI):

(wherein R², R⁴ and R⁵ each independently represent a C1-12 hydrocarbongroup, and R³ represents a C1-12 straight-chain or branched alkylenegroup).

The secondary alcohol (cb4) is a compound represented by formula (VI)above. Of these, R² is preferably a methyl group from the viewpoint ofincreasing reactivity between the active methylene-type blockedpolyisocyanate compound (cb3) and the secondary alcohol (cb4). Also, ifR³, R⁴ and R⁵ each have high carbon numbers, the polarity of theobtained active methylene-type blocked polyisocyanate compound (c3) maybe reduced and compatibility with the other coating material componentsmay be lowered, and therefore R³ is preferably a C1-3 alkylene group,and R⁴ and R⁵ are preferably methyl groups.

Examples for the secondary alcohol (cb4) include 4-methyl-2-pentanol,5-methyl-2-hexanol, 6-methyl-2-heptanol and 7-methyl-2-octanol, any ofwhich may be used alone or in combinations of two or more. Of these,4-methyl-2-pentanol which has a relatively low boiling point is morepreferred, since this will help facilitate removal of the secondaryalcohol (cb4) when some or all of the unreacted secondary alcohol (cb4)is distilled off following reaction between the blocked polyisocyanatecompound (cb3) and the secondary alcohol (cb4).

Specifically, the active methylene-type blocked polyisocyanate compound(c3) can be obtained, for example, by reacting an active methylene-typeblocked polyisocyanate compound (cb3-1) having a blocked isocyanategroup, represented by the following formula (IV):

(wherein each R⁴ independently represents a C1-12 hydrocarbon group, andmay be the same or different), mentioned above in explaining the activemethylene-type blocked polyisocyanate compound (cb3), with theaforementioned secondary alcohol (cb4).

In this case, at least one of the R⁴ groups in the blocked isocyanategroup in the active methylene-type blocked polyisocyanate compound(cb3-1) is replaced with a group represented by the following formula(VII):

(wherein R², R⁴ and R⁵ each independently represent a C1-12 hydrocarbongroup, and R³ represents a C1-12 straight-chain or branched alkylenegroup).

Also in this case, the active methylene-type blocked polyisocyanatecompound (c3) that is obtained has a blocked isocyanate grouprepresented by the following formula (I):

(wherein R¹, R², R⁴ and R⁵ each independently represent a C1-12hydrocarbon group, and R³ represents a C1-12 straight-chain or branchedalkylene group).

In formula (I), R⁴ is more preferably a C1-8 hydrocarbon group, evenmore preferably a C1-4 hydrocarbon group and most preferably a C2-3hydrocarbon group.

Also, the active methylene-type blocked polyisocyanate compound (c3)that is obtained has a blocked isocyanate group represented by thefollowing formula (II):

(wherein R², R³, R⁴ and R⁵ are as defined above).

The reaction between the active methylene-type blocked polyisocyanatecompound (cb3-1) and the secondary alcohol (cb4) is not particularlyrestricted so long as it is a method that, for example, can replace atleast one R⁴ group of the blocked isocyanate group in the activemethylene-type blocked polyisocyanate compound (cb3-1) with a grouprepresented by formula (VII) above. Preferred among such methods is amethod in which some or all of the alcohols derived from at least one R⁴group in the active methylene-type blocked polyisocyanate compound(cb3-1) are distilled out of the system by heating and pressurereduction or the like, and reaction is promoted to obtain an activemethylene-type blocked polyisocyanate compound (C3) having a blockedisocyanate group represented by formula (I) or (II) above.

Specifically, as the production method, some or all of the alcohol maybe removed by 5 minutes to 20 hours and preferably 10 minutes to 10hours at a temperature of 20° C. to 150° C. and preferably 75° C. to 95°C., under reduced pressure if necessary. If the temperature is too lowthe exchange reaction of the alkoxy groups in the active methylene-typeblocked polyisocyanate compound (cb3-1) will be slow, lowering theproduction efficiency, while if it is too high, the activemethylene-type blocked polyisocyanate compound (c3) may violentlydecompose and degrade, sometimes lowering the curability.

The active methylene-type blocked polyisocyanate compound (c3) can alsobe obtained by reacting an active methylene-type blocked polyisocyanatecompound (cb3-2) having a blocked isocyanate group, represented by thefollowing formula (V):

(wherein R⁶ and R⁷ each independently represent a C1-12 hydrocarbongroup),mentioned above in explaining the active methylene-type blockedpolyisocyanate compound (cb3), with the aforementioned secondary alcohol(cb4).

In this case, R⁷ of the blocked isocyanate groups in the activemethylene-type blocked polyisocyanate compound (cb3-2) is replaced witha group represented by the following formula (VII):

(wherein R², R⁴ and R⁵ each independently represent a C1-12 hydrocarbongroup, and R³ represents a C1-12 straight-chain or branched alkylenegroup).

In this case, the obtained active methylene-type blocked polyisocyanatecompound (c3) has a blocked isocyanate group represented by thefollowing formula (III):

(wherein R², R³, R⁴ and R⁵ are as defined above, and R⁶ represents aC1-12 hydrocarbon group).

The reaction between the active methylene-type blocked polyisocyanatecompound (cb3-2) and the secondary alcohol (cb4) is not particularlyrestricted so long as it is a method that, for example, can replace theR⁷ group of the blocked isocyanate groups in the active methylene-typeblocked polyisocyanate compound (cb3-2) with a group represented byformula (VII) above. Preferred among such methods is a method in whichsome or all of the alcohols derived from the R⁷ group in the activemethylene-type blocked polyisocyanate compound (cb3-2) are distilled outof the system by heating and pressure reduction or the like, andreaction is promoted to obtain an active methylene-type blockedpolyisocyanate compound (c3) having a blocked isocyanate grouprepresented by formula (III) above.

Specifically, as the production method, some or all of the alcohol maybe removed by 5 minutes to 20 hours and preferably 10 minutes to 10hours at a temperature of 20° C. to 150° C. and preferably 75° C. to 95°C., under reduced pressure if necessary. If the temperature is too lowthe exchange reaction of the alkoxy groups in the active methylene-typeblocked polyisocyanate compound (cb3-2) will be slow, lowering theproduction efficiency, while if it is too high, the activemethylene-type blocked polyisocyanate compound (c3) may violentlydecompose and degrade, sometimes lowering the curability.

Also, the mixing proportion of the active methylene-type blockedpolyisocyanate compound (cb3) and the secondary alcohol (cb4) forproduction of the active methylene-type blocked polyisocyanate compound(c3) is preferably in the range of 5 to 500 parts by mass and morepreferably in the range of 10 to 200 parts by mass for the secondaryalcohol (cb4), based on 100 parts by mass of solid content of the activemethylene-type blocked polyisocyanate compound (cb3), from the viewpointof reactivity of the active methylene-type blocked polyisocyanatecompound (c3) that is to be obtained, and production efficiency. At lessthan 5 parts by mass, the reaction rate between the activemethylene-type blocked polyisocyanate compound (cb3) and the secondaryalcohol (cb4) will be too slow. Also, if it is greater than 500 parts bymass, the concentration of the active methylene-type blockedpolyisocyanate compound (C3) will be too low, potentially lowering theproduction efficiency.

Also, for the reaction between the active methylene-type blockedpolyisocyanate compound (cb3) and the secondary alcohol (cb4), theremoval procedure described above may be carried out after adding thepolyfunctional hydroxyl group-containing compound to the activemethylene-type blocked polyisocyanate compound (cb3) and secondaryalcohol (cb4), in order to adjust the molecular weight of the activemethylene-type blocked polyisocyanate compound (c3).

The number-average molecular weight of the active methylene-type blockedpolyisocyanate compound (c3) is preferably in the range of 600 to30,000, from the viewpoint of compatibility with the other coatingmaterial components, and the smoothness, distinctness of image, waterresistance and chipping resistance of the multilayer coating film thatis to be formed. The upper limit for the number-average molecular weightis more preferably 10,000 and even more preferably 5,000 from theviewpoint of compatibility with the other coating material componentsand smoothness and distinctness of image of the multilayer coating filmthat is to be formed. The lower limit is more preferably 900 and evenmore preferably 1,000, from the viewpoint of the water resistance andchipping resistance of the multilayer coating film that is to be formed.

The multilayer coating film formed using a water-based coatingcomposition containing the active methylene-type blocked polyisocyanatecompound (c3) has satisfactory smoothness, distinctness of image andwater resistance, and also excellent curability at low temperature (70°C. or higher and below 120° C.). The reason for the satisfactorysmoothness and distinctness of image is conjectured to be that theactive methylene-type blocked polyisocyanate compound (c3) has ahydrocarbon group with a specific branched structure, and therefore whenthe water-based second pigmented coating material (B) is applied ontothe uncured first pigmented coating film obtained by applying thewater-based first pigmented coating material (A), to obtain the secondpigmented coating film, layer mixing between the coating films isinhibited. In addition, since the active methylene-type blockedpolyisocyanate compound (c3′) with a hydrophilic group often has highstorage stability, the curability and water resistance after long-termstorage are satisfactory.

The water-based coating composition of the invention includes an acrylicresin emulsion (a), coating film-forming resin (b) and curing agent (c)at 7 to 60 mass %, 10 to 60 mass % and 10 to 50 mass %, preferably 10 to60 mass %, 10 to 60 mass % and 10 to 50 mass %, more preferably 10 to 55mass %, 15 to 55 mass % and 15 to 45 mass %, even more preferably 15 to55 mass %, 15 to 55 mass % and 15 to 45 mass %, yet more preferably 20to 50 mass %, 20 to 50 mass % and 20 to 40 mass % and even yet morepreferably 12 to 50 mass %, 20 to 50 mass % and 20 to 40 mass %,respectively, based on the total mass of the solid resin content in thewater-based coating composition. This is from the viewpoint of improvingthe smoothness, distinctness of image, water resistance and chippingresistance of the multilayer coating film.

The water-based coating composition of the invention may be a one-packtype coating material, or it may be a multipack coating material used bymixing a base compound including an acrylic resin emulsion (a) and acoating film-forming resin (b), and a curing agent (c) and othercomponents, prior to coating.

Also, from the viewpoint of smoothness and distinctness of image of thecoating film that is to be obtained, the coating film absorptionpercentage, of the predried coating film of the water-based coatingcomposition of the invention, is in the range of no greater than 40 mass%, preferably 12 to 40 mass % and even more preferably 16 to 38 mass %.

The coating film absorption percentage can be calculated, for example,by the following test method and formula.

The water-based first pigmented coating material (A) is spray coatedonto two aluminum foils: aluminum foil A and aluminum foil B, to a dryfilm thickness of 20 μm. These are predried at 80° C. for 5 minutes, andthe aluminum foil B is immersed in purified water for 2 minutes.Aluminum foils A and B are then dried at 140° C. for 20 minutes.

The masses and nonvolatile contents of the aluminum foils are specifiedfor this series of steps, and the coating film absorption percentage iscalculated.

-   -   Initial mass of aluminum foil A: α0    -   Mass of aluminum foil A after predrying: α1    -   Mass of aluminum foil A after baking: α3    -   Nonvolatile content (%) of aluminum foil A after predrying: α4

α4=[(α3−α0)/(α1−α0)]×100

-   -   Initial mass of aluminum foil B: β0    -   Mass of aluminum foil B after predrying: β1    -   Mass of aluminum foil B after water soaking: β2    -   Mass of aluminum foil B after baking: β3    -   Nonvolatile content (%) of coating film on aluminum foil B after        water soaking: β5

β5=[(β3−β0)/(β2−β0)]×100

-   -   Coating film absorption percentage (%): γ

γ=[1−(β5/α4)]×100

In order to control the coating film absorption percentage to no greaterthan 40 mass %, the acrylic resin emulsion (a) in the water-basedcoating composition of the invention has a core-shell structure wherethe core section is crosslinked, and it is effective for the hydroxylvalue of the core section and shell section to be in the range of 50 to200 mgKOH/g. Furthermore, it is effective if the acid value of theacrylic resin emulsion (a) is in the range of 5 to 25 mgKOH/g, the glasstransition temperature (Tg) of the core section is 20° C. or higher, thehydrophobic monomer is copolymerized in the range of 30 to 90 mass % inthe core section, and a blocked polyisocyanate compound (c2) is presentas the curing agent (c) of the water-based coating composition. As aresult, layer mixing between the first pigmented coating film and thesecond pigmented coating film is inhibited, and it is possible to obtaina multilayer coating film having more excellent smoothness, distinctnessof image and flip-flop properties.

The water-based coating composition of the invention may comprise acolor pigment and/or an extender pigment commonly used in the field ofcoating materials. Examples of color pigments include titanium dioxide,zinc oxide, zinc phosphate, aluminum phosphate, zinc molybdate, calciummolybdate, Prussian blue, ultramarine blue, cobalt blue, copperphthalocyanine blue, indanthrone blue, synthetic yellow iron oxide,transparent iron oxide red (yellow), bismuth vanadate, titanium yellow,zinc yellow, monoazo yellow, isoindolinone yellow, metal complex azoyellow, quinophthalone yellow, benzimidazolone yellow, iron oxide red,monoazo red quinacridone red, azo lake (Mn salt), quinacridone magenta,anthanthrone orange, dianthraquinonyl red, perylene maroon, quinacridonemagenta, perylene red, diketopyrrolopyrrolechromium vermilion,chlorinated phthalocyanine green, brominated phthalocyanine green,pyrazolone orange, benzimidazolone orange, dioxazine violet, peryleneviolet and the like, and examples of extender pigments include barytapowder, barium sulfate, barium carbonate, calcium carbonate, gypsum,clay, white carbon, diatomaceous earth, talc, magnesium carbonate,alumina white, gloss white, mica powder and the like, any of which maybe used alone or in combinations of two or more.

Also, the water-based coating composition of the invention may furthercomprise, as necessary, common coating material additives such as lusterpigments, curing catalysts, ultraviolet absorbers, light stabilizers,antifoaming agents, plasticizers, organic solvents, surface controlagents, anti-settling agents, thickening agents and the like, eitheralone or in combinations of two or more.

Examples of luster pigments include aluminum, copper, zinc, brass,nickel, aluminum oxide, mica, titanium oxide- or iron oxide-coveredaluminum oxide, titanium oxide- or iron oxide-covered mica, glassflakes, hologram pigments and the like, and examples of curing catalystsinclude organometallic compounds such as tin octylate, dibutyltindi(2-ethyl hexanoate), dioctyltin di(2-ethylhexanoate), dioctyltindiacetate, dibutyltin dilaurate, dibutyltin oxide, monobutyltintrioctate, lead 2-ethylhexylate and zinc octylate.

Examples for the thickening agent include inorganic thickening agentssuch as silicates, metal silicates, montmorillonite and colloidalalumina; polyacrylic acid-based thickening agents such as copolymers of(meth)acrylic acids and (meth)acrylic acid esters and sodiumpolyacrylate; associative thickening agents having a hydrophilic portionand a hydrophobic portion in the molecule and exhibiting an effectivethickening effect when the hydrophobic portion is adsorbed onto thepigment in the coating material and/or the emulsion particle surfaces inaqueous medium, such that the hydrophobic portions become associatedtogether; cellulosic derivative-based thickening agents such ascarboxymethyl cellulose, methyl cellulose and hydroxyethyl cellulose;protein-based thickening agents such as casein, sodium caseinate andammonium caseinate; alginic acid-based thickening agents such as sodiumalginate; polyvinyl-based thickening agents such as polyvinyl alcohol,polyvinylpyrrolidone and polyvinylbenzyl ether copolymer;polyether-based thickening agents such as Pluronic polyethers, polyetherdialkyl esters, polyether dialkyl ethers and polyether epoxy-modifiedcompounds; maleic anhydride copolymer-based thickening agents such aspartial esters of vinyl methyl ether-maleic anhydride copolymers; andpolyamide-based thickening agents such as polyamide amine salts. Thesethickening agents may be used alone or in combinations of two or more.

Commercial products may be used as polyacrylic acid-based thickeningagents. Examples of trade names of commercial products include “PRIMALASE-60”, “PRIMAL TT-615” and “PRIMAL RM-5” by Rohm & Haas, and “SNTHICKENER 613”, “SN THICKENER 618”, “SN THICKENER 630”, “SN THICKENER634” and “SN THICKENER 636” by San Nopco, Ltd.

A commercial product may also be used as the aforementioned associativethickening agent. Examples of trade names of commercial products include“UH-420”, “UH-450”, “UH-462”, “UH-472”, “UH-540”, “UH-752”, “UH-756VF”and “UH-814N” by Adeka Corp., “PRIMAL RM-8W”, “PRIMAL RM-825”, “PRIMALRM-2020NPR”, “PRIMAL RM-12W” and “PRIMAL SCT-275” by Rohm & Haas, and“SN THICKENER 612”, “SN THICKENER 621N”, “SN THICKENER 625N”, “SNTHICKENER 627N” and “SN THICKENER 660T” by San Nopco, Ltd.

The thickening agent is preferably a polyacrylic acid-based thickeningagent and/or associative thickening agent, with associative thickeningagents being more preferably used, and urethane associative thickeningagents having hydrophobic groups on the ends and containing urethanebonds in the molecular chain being even more preferably used. Commercialproducts may be used as urethane associative thickening agents. Examplesof trade names of commercial products include “UH-420”, “UH-462”,“UH-472”, “UH-540”, “UH-752”, “UH-756VF” and “UH-814N” by Adeka Corp.,and “SN THICKENER 612”, “SN THICKENER 621N”, “SN THICKENER 625N”, “SNTHICKENER 627N” and “SN THICKENER 660T” by San Nopco, Ltd.

Also, when the water-based coating composition of the inventioncomprises the aforementioned thickening agent, the thickening agentcontent is preferably 0.01 to 10 parts by mass, more preferably 0.02 to3 parts by mass and even more preferably 0.03 to 2 parts by mass, withrespect to 100 parts by mass as the total solid resin content.

[Article to be Coated]

Base materials to which the water-based coating composition of theinvention may be applied are not particularly restricted, and examplesinclude metals such as iron, aluminum, brass, copper, tin, stainlesssteel, galvanized steel and alloyed zinc (such as Zn—Al, Zn—Ni andZn—Fe)-plated steel and the like; resins such as polyethylene resins,polypropylene resins, acrylonitrile-butadiene-styrene (ABS) resins,polyamide resins, acrylic resins, vinylidene chloride resins,polycarbonate resins, polyurethane resins and epoxy resins, plasticmaterials such as various FRP materials; inorganic materials such asglass, cement and concrete; wood materials; fiber materials (such aspaper and fabrics), and the like, among which metals and plasticmaterials are preferred.

Also, there are no particular restrictions on articles to be coated byapplication of the water-based coating composition of the invention, andthere may be mentioned external platings of automobile bodies ofpassenger vehicles, trucks, motorcycles and buses; automobile parts; andexternal platings of consumer electric products such as cellular phonesor audio devices, among which external platings of automobile bodies andautomobile parts are preferred.

The article to be coated may also be an article that is a metal basematerial or has a metal surface such as a car body formed thereof, andthat has been surface-treated by phosphate treatment, chromate treatmentor complex oxide treatment. Furthermore, the article to be coated may beone having an undercoat film of any of various electrodepositioncoatings formed on the aforementioned metal base material, car body orthe like, and most preferably it is a car body having an undercoat filmformed by a cationic electrodeposition coating.

[Step (1)]

The water-based coating composition of the invention may be applied as awater-based first pigmented coating material (A) to an article to becoated by a known method such as air spraying, airless spraying, rotaryatomizing coating or the like, and electrostatic application may becarried out during the coating. The coating film thickness may be in arange of usually 10 to 100 μm, preferably 10 to 50 μm and morepreferably 15 to 35 μm, as the dry film thickness.

[Step (2)]

A water-based second pigmented coating material (B) may be furtherapplied onto the first pigmented coating film that has been formed inthis manner.

From the viewpoint of prevention of dripping and smoothness of thecoating film that is to be obtained, application of the water-basedsecond pigmented coating material (B) is preferably carried out whilethe solid content of the first pigmented coating film is in the range of70 to 100 mass %, preferably 80 to 100 mass % and more preferably 90 to100 mass %, and from the viewpoint of smoothness of the coating filmthat is to be obtained, it is preferably carried out while the gelfraction of the first pigmented coating film is 0 to 30 mass %,preferably 0.01 to 20 mass % and more preferably 1 to 15 mass %.

The solid content and gel fraction of the first pigmented coating filmcan be adjusted, for example, by predrying (preheating) and/or airblowing or the like, before application of the water-based secondpigmented coating material (B).

The predrying can usually be accomplished by direct or indirect heatingof the coated article in a drying furnace for about 1 to 30 minutes at atemperature of about 50° C. to about 110° C. and preferably about 60° C.to about 90° C., while air blowing can usually be accomplished byblasting air at ordinary temperature or heated to a temperature of about25° C. to about 80° C. onto the coated surface of the article.

The solid content of the first pigmented coating film is measured by thefollowing method.

First, the water-based coating composition of the invention is appliedonto an aluminum foil whose mass (W₁) has been measured beforehand,simultaneously with the article to be coated. Next, the aluminum foilthat has been coated and subjected to predrying as necessary isretrieved just before the water-based second pigmented coating material(B) is applied, and the mass (W₂) is measured. The retrieved aluminumfoil is then dried at 110° C. for 60 minutes and allowed to cool to roomtemperature in a desiccator, after which the mass (W₃) of the aluminumfoil is measured and the solid content is calculated by the followingformula.

Solid content (mass %)={(W ₃ −W ₁)/(W ₂ −W ₁)}×100

The gel fraction of the first pigmented coating film is measured by thefollowing method.

First, the water-based coating composition of the invention is appliedonto a polypropylene sheet simultaneously with the article to be coated,and the polypropylene sheet that has been coated and subjected topredrying as necessary, is retrieved just before the water-based secondpigmented coating material (B) is applied.

Next, the first pigmented coating film on the polypropylene sheet isretrieved and its mass (W_(a)) is measured. The coating film is thenplaced in a 200 mesh stainless steel mesh-like container, and extractedwhile circulating for 5 hours in acetone heated to 64° C., and afterdrying at 110° C. for 60 minutes, the coating film mass (W_(b)) wasmeasured and the insoluble coating film residue rate (mass %) obtainedby the following formula was recorded as the gel fraction.

Gel fraction (mass %)=(W _(b) /W _(a))×100

As the water-based second pigmented coating material (B) there may beused any known one commonly used for coating of automobile bodies, forexample. Specifically, there may be used one having a base resin such asan acrylic resin, polyester resin, alkyd resin, urethane resin or epoxyresin having crosslinkable functional groups such as carboxyl andhydroxyl, and a resin component composed of a crosslinking agent such asan optionally blocked polyisocyanate compound, melamine resin or urearesin, dissolved or dispersed in water together with a pigment and otheradditives, to form a coating material. Preferred among these arethermosetting water-based coating materials containing hydroxylgroup-containing resins and melamine resins.

The pigment component used may be any of the color pigments, lusterpigments, extender pigments or the like mentioned above as examples inexplaining the water-based coating composition of the invention, and byusing a luster pigment as at least one type of pigment component it ispossible to form a metallic-like or pearl-like coating film withcompactness.

The water-based second pigmented coating material (B) may be applied bya known method such as air spraying, airless spraying, rotary atomizingcoating or the like, and electrostatic application may be carried outduring the coating. The coating film thickness may be in the range ofgenerally 5 to 40 μm and preferably 10 to 30 μm, as the cured filmthickness.

The coating film of the applied water-based second pigmented coatingmaterial (B) may be dried by heating for about 1 to 60 minutes at atemperature of about 50° C. to about 110° C. and preferably about 60° C.to about 90° C., using predrying (preheating) and/or air blowing afterapplication.

[Step (3)]

A clear coating material may be further applied onto the first pigmentedcoating film and second pigmented coating film formed in the mannerdescribed above.

The clear coating material used may be any known one commonly used forcoating of automobile bodies, for example. Specifically, there may bementioned organic solvent-based thermosetting coating materials, aqueousthermosetting coating materials and thermosetting powder coatingmaterials comprising, as resin components, base resins such as acrylicresins, polyester resins, alkyd resins, urethane resins, epoxy resinsand fluorine resins, that have crosslinkable functional groups such ashydroxyl, carboxyl, epoxy or silanol, and crosslinking agents such asmelamine resins, urea resins, non-blocked polyisocyanate compounds,carboxyl group-containing compounds or resins and epoxy group-containingcompounds or resins. Preferred among these are thermosetting coatingmaterials comprising a carboxyl group-containing resin and an epoxygroup-containing resin, thermosetting coating materials comprising ahydroxyl group-containing resin and a melamine resin, and thermosettingcoating materials comprising a hydroxyl group-containing resin and anon-blocked polyisocyanate compound.

The clear coating material may be a one-pack type coating material or itmay be a two-pack coating material such as a two-pack urethane resincoating material.

Also, the clear coating material may comprise, if necessary, colorpigments, luster pigments, dyes and the like in ranges that do notimpair the transparency, and may further contain, as appropriate,extender pigments, ultraviolet absorbers, antifoaming agents, thickeningagents, rust-preventive agents, surface control agents and the like.

The clear coating material may be applied onto the coating film surfaceof the water-based second pigmented coating material (B) by a knownmethod such as air spraying, airless spraying, rotary atomizing coatingor the like, and electrostatic application may be carried out during thecoating. The clear coating material may be applied to a dry filmthickness in the range of 10 to 60 μm and preferably 20 to 50 μm.

[Step (4)]

The multilayer coating film composed of a three-layer coating filmconsisting of the first pigmented coating film, second pigmented coatingfilm and clear coating film may be simultaneously cured by heating forabout 20 to about 40 minutes at a temperature of about 80° C. to about170° C. and preferably about 120° C. to about 160° C., by hot airheating, infrared heating, high-frequency heating or the like, forexample, using common coating film baking means.

EXAMPLES

The present invention will now be described in greater detail byexamples, with the understanding that the invention is not limited onlyto these examples. The “parts” and “%” values refer to “parts by mass”and “mass %”.

Production of Acrylic Resin Emulsion (a) Production Example 1

Into a reactor equipped with a thermometer, thermostat, stirrer, refluxcondenser, nitrogen inlet tube and dropper there were charged 130 partsof deionized water and 0.52 part of “AQUALON KH-10” (trade name ofDai-ichi Kogyo Seiyaku Co., Ltd., ammonium salt of polyoxyethylenealkylether sulfate, active ingredient: 97%), and these were stirred and mixedunder a nitrogen stream and heated to 80° C. Next, 1% of the totalmonomer emulsion (1) described below and 5.3 parts of a 6% ammoniumpersulfate aqueous solution were introduced into the reactor, and themixture was kept at 80° C. for 15 minutes. The remainder of the monomeremulsion (1) was then added dropwise into the reactor kept at the sametemperature over a period of 3 hours, and upon completion of thedropwise addition the mixture was aged for 1 hour.

Next, the monomer emulsion (2) described below was added dropwise over aperiod of 1 hour and aged for 1 hour, and then cooled to 30° C. whilegradually adding 40 parts of a 5% 2-(dimethylamino)ethanol aqueoussolution to the reactor and discharged while filtering with a 100 meshnylon cloth, to obtain an aqueous dispersion of an acrylic resinemulsion (a-1) having a mean particle diameter of 118 nm (measured at20° C. using a “COULTER N4” submicron particle size distributionanalyzer (product of Beckman Coulter, Inc.) after dilution withdeionized water) and a solid concentration of 30%. The obtained acrylicresin emulsion had a hydroxyl value of 65 mgKOH/g (core copolymerhydroxyl value: 65 mgKOH/g, shell copolymer hydroxyl value: 65 mgKOH/g),an acid value of 13 mgKOH/g and a glass transition temperature of 37° C.

Monomer Emulsion (1):

Monomer emulsion (1) was obtained by mixing and stirring 46.2 parts ofdeionized water, 0.79 part of “AQUALON KH-10”, 7.7 parts of styrene,16.94 parts of methyl methacrylate, 7.7 parts of n-butyl acrylate, 30.8parts of n-butyl methacrylate, 11.55 parts of 2-hydroxyethylmethacrylate and 2.31 parts of ethyleneglycol dimethacrylate. (Total ofcore copolymer-forming monomers: 77 parts)

Monomer Emulsion (2):

Monomer emulsion (2) was obtained by mixing and stirring 13.8 parts ofdeionized water, 0.24 part of “AQUALON KH-10”, 0.03 part of ammoniumpersulfate, 2.3 parts of styrene, 6.9 parts of methyl methacrylate, 4.6parts of ethyl acrylate, 3.68 parts of n-butyl acrylate, 3.45 parts of2-hydroxyethyl methacrylate and 2.07 parts of methacrylic acid. (Totalof shell copolymer-forming monomers: 23 parts)

Production Examples 2 to 20

Acrylic resin emulsions (a-2) to (a-20) were obtained in the same manneras Production Example 1, except for changing the compositions of monomeremulsions (1) and (2) as shown in Table 1 below.

Table 1 shows the monomer compositions (parts), hydroxyl values(mgKOH/g), acid values (mgKOH/g) and glass transition temperatures(Tg)(° C.) of acrylic resin emulsions (a-1) to (a-20). The monomercompositions in the table are all values for the monomer components.

TABLE 1 Production Example No. Prod. Prod. Prod. Prod. Prod. Prod. Prod.Prod. Prod. Prod. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9Ex. 10 Acrylic resin emulsion (a) No. a-1 a-2 a-3 a-4 a-5 a-6 a-7 a-8a-9 a-10 Core section St 7.7 7.7 9.2 5.4 9.2 6.2 9.0 6.0 6.2 7.7copolymer (I) MMA 16.9 9.2 15.4 23.1 15.4 18.3 19.8 13.2 32.3 24.8 EA0.0 0.0 1.5 0.0 0.0 0.0 0.0 0.0 0.0 19.3 nBA 7.7 7.7 6.2 11.6 9.2 7.718.0 9.0 9.2 3.9 nBMA 30.8 23.1 30.0 11.6 29.3 31.6 27.0 20.4 15.4 7.72EHA 0.0 15.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 HEA 0.0 0.0 0.0 11.6 0.00.0 0.0 0.0 0.0 0.0 HEMA 11.6 11.6 11.6 11.6 11.6 10.8 13.5 9.6 11.611.6 AMA 0.0 0.0 2.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 EGDMA 2.3 2.3 0.8 2.32.3 2.5 2.7 1.8 2.3 2.2 Total 77 77 77 77 77 77 90 60 77 77 Shellsection St 2.3 3.5 2.3 2.8 2.8 2.3 1.0 4.0 2.3 3.5 copolymer (II) MMA6.9 10.4 6.4 6.4 4.1 2.3 2.0 13.6 11.5 5.8 EA 4.6 2.3 4.1 4.6 3.5 1.42.0 8.0 2.2 4.6 nBA 3.7 1.4 4.1 3.6 3.7 6.9 2.1 6.4 1.4 3.7 nBMA 0.0 0.00.5 0.0 0.0 4.6 0.0 0.0 0.0 0.0 HEA 0.0 0.0 0.0 0.0 3.5 0.0 0.0 0.0 0.00.0 HEMA 3.5 3.5 3.5 3.5 3.5 3.5 1.5 6.0 3.6 3.5 MAAc 2.1 2.0 2.1 2.12.1 2.1 1.4 2.0 2.1 2.1 EGDMA 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Total 23 23 23 23 23 23 10 40 23 23 Core/shell ratio (solid mass ratio)77/23 77/23 77/23 77/23 77/23 77/23 90/10 60/40 77/23 77/23 Hydroxylvalue Core section copolymer 65 65 65 137 65 60 65 69 65 65 (mgKOH/g)Shell section copolymer 65 65 65 65 137 65 65 65 67 65 Overall 65 65 65121 81 61 65 67 65 65 Acid value (mgKOH/g) 13 13 13 14 13 13 9 13 13 13Glass transition temperature (° C.) 37 22 37 22 30 31 26 33 54 36Production Example No. Prod. Prod. Prod. Prod. Prod. Prod. Prod. Prod.Prod. Prod. Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Ex.19 Ex. 20 Acrylic resin emulsion (a) No. a-11 a-12 a-13 a-14 a-15 a-16a-17 a-18 a-19 a-20 Core section St 9.2 6.2 10.0 9.2 9.2 7.7 3.9 7.7 7.77.7 copolymer (I) MMA 15.4 18.5 25.0 15.4 15.4 16.9 10.8 17.7 15.4 16.9EA 0.0 0.0 2.0 0.0 0.0 0.0 0.0 1.5 0.0 0.0 nBA 7.7 15.4 10.0 7.7 7.711.6 7.7 7.7 7.7 7.7 nBMA 30.8 15.4 35.0 36.2 36.2 27.0 13.9 30.8 25.430.8 2EHA 0.0 7.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 HEA 0.0 0.0 3.0 0.00.0 0.0 0.0 0.0 0.0 0.0 HEMA 11.6 11.6 15.0 6.2 6.2 11.6 38.5 11.6 11.611.6 AMA 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 9.2 0.0 EGDMA 2.3 2.3 0.0 2.32.3 2.3 2.3 0.0 0.0 2.3 Total 77 77 100 77 77 77 77 77 77 77 Shellsection St 1.8 2.3 0.0 2.3 2.3 2.3 3.5 2.3 2.3 2.3 copolymer (II) MMA4.6 4.6 0.0 6.9 4.6 8.5 4.6 5.8 6.4 6.4 EA 5.1 6.9 0.0 4.6 3.5 4.6 5.85.8 4.6 4.6 nBA 3.2 3.7 0.0 3.7 3.7 3.7 3.7 3.5 3.6 3.7 nBMA 0.9 0.0 0.00.0 0.0 0.0 0.0 0.0 0.5 0.0 HEA 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0HEMA 3.5 3.5 0.0 3.5 6.9 1.8 3.5 3.7 3.5 3.5 MAAc 3.9 2.1 0.0 2.1 2.12.1 2.1 2.1 2.1 2.1 EGDMA 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 Total23 23 0 23 23 23 23 23 23 23 Core/shell ratio (solid mass ratio) 77/2377/23 100/0 77/23 77/23 77/23 77/23 77/23 77/23 77/23 Hydroxyl valueCore section copolymer 65 65 79 35 35 65 216 65 65 65 (mgKOH/g) Shellsection copolymer 65 65 0 65 130 35 65 69 65 65 Overall 65 65 79 42 5658 181 66 65 65 Acid value (mgKOH/g) 26 13 20 13 13 13 13 13 14 13 Glasstransition temperature (° C.) 38 18 42 35 35 33 38 35 37 37

The abbreviations in Table 1 have the following meanings.

St: Styrene

MMA: Methyl methacrylateEA: Ethyl acrylatenBA: n-Butyl acrylatenBMA: n-Butyl methacrylateHEA: 2-Hydroxyethyl acrylateHEMA: 2-Hydroxyethyl methacrylateMAAc: Methacrylic acidAMA: Allyl methacrylateEGDMA: Ethyleneglycol dimethacrylate

Production of Coating Film-Forming Resin (b1) Production Example 21

Into a reactor equipped with a thermometer, thermostat, stirrer, refluxcondenser and water separator there were charged 142 parts ofisophthalic acid, 313 parts of adipic acid, 59 parts ofmethylpentanediol, 220 parts of trimethylolpropane and 228 parts ofbutylethylpropanediol, and after heating from 160° C. to 230° C. over aperiod of 3 hours, the condensation water produced was distilled offwith a water separator while maintaining a temperature of 230° C., andreaction was conducted until the acid value fell below 3 mgKOH/g. Tothis reaction product there was added 38 parts of trimellitic anhydride,and after addition reaction at 170° C. for 30 minutes, it was cooled tobelow 50° C., 2-(dimethylamino)ethanol was added in an amount equivalentto the acid groups for neutralization, and then deionized water wasslowly added to obtain a hydroxyl group-containing polyester resin(b1-1) solution with a solid concentration of 45% and at pH 6.3. Theacid value of the obtained hydroxyl group-containing polyester resin(b1-1) was 35 mgKOH/g, the hydroxyl value was 164 mgKOH/g and thenumber-average molecular weight was 1600.

Production of Coating Film-Forming Resin (b2) Production Example 22

Into a reactor equipped with a thermometer, thermostat, stirrer, refluxcondenser, nitrogen inlet tube and dropper there was charged 30 parts ofpropyleneglycol monopropyl ether, and after raising the temperature to95° C., a mixture of 10 parts of styrene, 30 parts of methylmethacrylate, 25 parts of 2-ethylhexyl acrylate, 11.5 parts of n-butylacrylate, 20 parts of hydroxyethyl acrylate, 3.5 parts of acrylic acid,10 parts of propyleneglycol monopropyl ether and 2 parts of2,2′-azobis(2,4-dimethylvaleronitrile) was added dropwise over a periodof 4 hours, and upon completion of the dropwise addition the mixture wasaged for 1 hour. Next, a mixture of 5 parts of propyleneglycolmonopropyl ether and 1 part of 2,2′-azobis(2,4-dimethylvaleronitrile)was further added dropwise into a flask over a period of 1 hour, andupon completion of the dropwise addition the mixture was aged for 1hour. There was further added 3.03 parts of 2-(dimethylamino)ethanol,and deionized water was slowly added thereto to obtain a solution of ahydroxyl group-containing water-soluble acrylic resin (b2-1) with asolid concentration of 40%. The acid value of the obtained hydroxylgroup-containing acrylic resin was 27 mgKOH/g, the hydroxyl value was 97mgKOH/g, and the weight-average molecular weight was approximately50,000.

Production of Active Methylene-Type Blocked Polyisocyanate Compound (c3)Production Example 23

After charging 360 parts of “SUMIDUR N-3300”, 60 parts of “UNIOX M-550”(product of NOF Corp., polyethyleneglycol monomethyl ether, averagemolecular weight: approximately 550) and 0.2 part of2,6-di-tert-butyl-4-methylphenol into a reactor equipped with athermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube,dropper and simple removed solvent trap, the contents were thoroughlymixed and heated at 130° C. for 3 hours under a nitrogen stream. Next,110 parts of ethyl acetate and 252 parts of diisopropyl malonate werecharged in, 3 parts of a 28% methanol solution of sodium methoxide wasadded while stirring under a nitrogen stream, and stirring was continuedat 65° C. for 8 hours. The isocyanate content in the obtained resinsolution was 0.12 mol/Kg. After adding 683 parts of 4-methyl-2-pentanolto this, the solvent was distilled off over a period of 3 hours underreduced pressure conditions, while keeping the system temperature at 80°C. to 85° C., to obtain 1010 parts of a solution of an activemethylene-type blocked polyisocyanate compound (c3-1). The simple trapfor the removed solvent included 95 parts of isopropanol. The solidconcentration of the obtained solution of the active methylene-typeblocked polyisocyanate compound (c3-1) was approximately 60%.

Production Example 24

After charging 480 parts of “SUMIDUR N-3300” (trade name of Sumika BayerUrethane Co., Ltd., polyisocyanate containing isocyanurate structurederived from hexamethylene diisocyanate, solid content: approximately100%, isocyanate group content: 21.8%), 150 parts of ethyl acetate and365 parts of diisopropyl malonate into a reactor equipped with athermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube,dropper and simple removed solvent trap, 4 parts of a 28% sodiummethoxide methanol solution was added while stirring under a nitrogenstream and the mixture was stirred at 65° C. for 8 hours. The isocyanatecontent in the obtained resin solution was 0.07 mol/Kg. To this therewas added 870 parts of 4-methyl-2-pentanol, the solvent was distilledoff over a period of 3 hours under reduced pressure conditions whilemaintaining a system temperature of 90° C. to 95° C., and 120 parts of4-methyl-2-pentanol was further added to obtain 1400 parts of a solutionof an active methylene-type blocked polyisocyanate compound (c3-2). Thesimple trap for the removed solvent included 183 parts of isopropanol.The solid concentration of the obtained solution of the activemethylene-type blocked polyisocyanate compound (c3-2) was approximately60%.

Production Example 25

After charging 480 parts of “SUMIDUR N-3300”, 150 parts of ethyl acetateand 310 parts of diethyl malonate into a reactor equipped with athermometer, thermostat, stirrer, reflux condenser, nitrogen inlet tube,dropper and simple removed solvent trap, 4 parts of a 28% sodiummethoxide methanol solution was added while stirring under a nitrogenstream and the mixture was stirred at 65° C. for 8 hours. The isocyanatecontent in the obtained resin solution was 0.06 mol/Kg. To this therewas added 630 parts of n-butanol, the solvent was distilled off over aperiod of 3 hours under reduced pressure conditions while maintaining asystem temperature of 90° C. to 95° C., and 90 parts of n-butanol wasadded to obtain 1270 parts of a solution of an active methylene-typeblocked polyisocyanate compound (c3-3). The simple trap for the removedsolvent included 100 parts of ethanol. The solid concentration of theobtained solution of the active methylene-type blocked polyisocyanatecompound (c3-3) was approximately 60%.

Production of Water-Based First Pigmented Coating Material (A) Example 1

After mixing 25 parts of the solution of the hydroxyl group-containingwater-soluble acrylic resin (b2-1) obtained in Production Example 22(solid resin content: 10 parts), 60 parts of “JR-806” (trade name ofTayca Corp., rutile titanium dioxide), 1 part of “CARBON MA-100” (tradename of Mitsubishi Chemical Corp., carbon black), 15 parts of “BARIACEB-35” (trade name of Sakai Chemical Industry Co., Ltd., barium sulfatepowder, mean primary particle size: 0.5 μm), 3 parts of “MICRO ACE S-3”(trade name of Nippon Talc Co., Ltd., talc powder, mean primary particlesize: 4.8 μm) and 46 parts of deionized water, and adjusting the mixtureto pH 8.0 with 2-(dimethylamino)ethanol, it was dispersed for 30 minuteswith a paint shaker to obtain a pigment-dispersed paste.

Next, 150 parts of the obtained pigment-dispersed paste (solid resincontent: 10 parts), 100 parts of the acrylic resin emulsion (a-1)dispersion obtained in Production Example 1 (solid resin content: 30parts), 44.4 parts of the hydroxyl group-containing polyester resin(b1-1) solution obtained in Production Example 21 (solid resin content:20 parts), 28.6 parts of (UCOAT UX-8100″ (trade name of Sanyo ChemicalIndustries, Ltd., urethane emulsion, solid content: 35%) (solid resincontent: 10 parts), 25 parts of the blocked polyisocyanate compound(c3-1) solution obtained in Production Example 23 (solid resin content:15 parts), 15 parts of the melamine resin “SAIMEL 350” (trade name ofMitsui Saitech Co., Ltd., methyl etherified melamine resin,weight-average molecular weight: 550, solid content: 100%) (solid resincontent: 15 parts) and 10 parts of 2-ethyl-1-hexanol were uniformlymixed.

To the obtained mixture there was then added “UH-752” (trade name ofAdeka Corp., urethane associative thickening agent),2-(dimethylamino)ethanol and deionized water, to obtain water-basedfirst pigmented coating material (A-1) having pH 8.0, a solidconcentration of 48%, and a viscosity of 30 seconds with a Ford cup No.4 at 20° C.

Examples 2 to 17, Comparative Examples 1 to 11

Water-based first pigmented coating materials (A-2) to (A-28), eachhaving pH 8.0 and a viscosity of 30 seconds with a Ford cup No. 4 at 20°C., were obtained in the same manner as Example 1, except that themixing compositions were as shown in Table 2 below. The contents in thetable are all solid contents.

TABLE 2 Example No. Exam- Exam- Exam- Exam- Example 1 Example 2 Example3 Example 4 Example 5 ple 6 ple 7 ple 8 ple 9 Water-based firstpigmented coating material A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 (A) No.Acrylic resin emulsion (a) No. a-1 a-2 a-3 a-4 a-5 a-6 a-7 a-8 a-9Amount 30 32 30 34 30 32 30 28 30 Coating Polyester resin b1-1 20 16 2216 20 20 18 19 18 film- (b1) forming Water-soluble b2-1 10 12 10 10 1210 10 13 11 resin (b) acrylic resin (b2) Urethane resin UX-8100 10 10  810  8  8 12 10 11 (b3) Total coating film-forming resin 40 38 40 36 4038 40 42 40 (b) amount Curing Melamine resin SAIMEL 350 15 14 15 15 1515 16 15 15 agent (c) (c1) SAIMEL 250 (*2) — — — — — — — — — Active c3-115 16 15 15 15 15 14 15 15 methylenic block c3-2 — — — — — — — — —polyisocyanate c3-3 — — — — — — — — — compound (c3) Total curing agent(c) amount 30 30 30 30 30 30 30 30 30 Total resin solid portion amount100  100  100  100  100  100  100  100  100  Example No. Example ExampleExample Example Example Example Example Example 10 11 12 13 14 15 16 17Water-based first pigmented coating material A-10 A-11 A-12 A-13 A-14A-15 A-16 A-17 (A) No. Acrylic resin emulsion (a) No. a-1 a-1 a-1 a-1a-10 a-11 a-12 a-1 Amount 35 30 30 30 30 30 30 10 Coating Polyesterresin b1-1 20 18 18 18 25 25 25 25 film- (b1) forming Water-soluble b2-115 12 12 12 15 15 15 15 resin (b) acrylic resin (b2) Urethane resin (b3)UX-8100 — 10 10 10 — — — 10 Total coating film-forming resin 35 40 40 4040 40 40 50 (b) amount Curing Melamine resin (c1) SAIMEL 350 — 30 15 1516 15 16 20 agent (c) SAIMEL 250 (*2) 15 — — — — — — — Active methylenicc3-1 15 — — — 14 15 14 20 block c3-2 — — 15 — — — — — polyisocyanatecompound (c3) c3-3 — — — 15 — — — — Total curing agent (c) amount 30 3030 30 30 30 30 40 Total resin solid portion amount 100  100  100  100 100  100  100  100  Comp. Ex. No. Comp. Comp. Comp. Comp. Comp. Comp.Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7Ex. 8 Ex. 9 Ex. 10 Ex. 11 Water-based first pigmented coating materialA-18 A-19 A-20 A-21 A-22 A-23 A-24 A-25 A-26 A-27 A-28 (A) No. Acrylicresin emulsion (a) No. a-13 a-14 a-15 a-16 a-17 a-18 a-19 a-20 a-1 a-1a-1 Amount 30 28 28 30 30 30 32 30 42 5 65 Coating Polyester resin b1-120 20 20 22 20 19 18 20 25 40 — film- (b1) forming Water-soluble b2-1 1010 10 10 10 11 10 10 15 15 10 resin (b) acrylic resin (b2) UrethaneUX-8100 10 12 12 8 10 10 10 10 10 10 — resin (b3) Total coatingfilm-forming resin 40 42 42 40 40 40 38 40 50 65 10 (b) amount CuringMelamine SAIMEL 350 15 15 15 13 17 15 15 15  4 15   12.5 agent (c) resin(c1) SAIMEL 250 — — — — — — — — — — — (*2) Active c3-1 15 15 15 17 13 1515 15 4 15   12.5 methylenic c3-2 — — — — — — — — — — — block c3-3 — — —— — — — — — — — polyisocyanate compound (c3) Total curing agent (c)amount 30 30 30 30 30 30 30 30  8 30 25 Total resin solid portion amount100  100  100  100  100  100  100  100  100  100  100 

(*1): SAIMEL 250:

Trade name of Mitsui Saitech Co., Ltd., methyl/butyl mixed etherifiedmelamine resin, number-average molecular weight: 2,700, solid content:70%.

Production of Acrylic Resin Emulsion for Water-Based Second PigmentedCoating Material (B) Production Example 26

After charging 130 parts of deionized water and 0.52 part of AQUALONKH-10 into a reactor equipped with a thermometer, thermostat, stirrer,reflux condenser, nitrogen inlet tube and dropper, the mixture wasstirred in a nitrogen stream and the temperature was increased to 80° C.Next, 1% of the total monomer emulsion (1) described below and 5.3 partsof a 6% ammonium persulfate aqueous solution were introduced into thereactor, and the mixture was kept at 80° C. for 15 minutes. Theremainder of the monomer emulsion (1) was then added dropwise into thereactor kept at the same temperature over a period of 3 hours, and uponcompletion of the dropwise addition the mixture was aged for 1 hour.Next, the monomer emulsion (2) described below was added dropwise over aperiod of 1 hour and aged for 1 hour, and then cooled to 30° C. whilegradually adding 40 parts of a 5% dimethylethanolamine aqueous solutionto the reactor and discharged while filtering with a 100 mesh nyloncloth, to obtain a dispersion of an acrylic resin emulsion (AC) having amean particle diameter of 100 nm (measured at 20° C. using a “COULTERN4” submicron particle size distribution analyzer (product of BeckmanCoulter, Inc.) after dilution with deionized water) and a solidconcentration of 30%. The obtained acrylic resin had an acid value of 33mgKOH/g and a hydroxyl value of 25 mgKOH/g.

Monomer Emulsion (1):

Monomer emulsion (1) was obtained by mixing and stirring 42 parts ofdeionized water, 0.72 part of “AQUALON KH-10”, 2.1 parts of methylenebisacrylamide, 2.8 parts of styrene, 16.1 parts of methyl methacrylate,28 parts of ethyl acrylate and 21 parts of n-butyl acrylate.

Monomer Emulsion (2):

Monomer emulsion (2) was obtained by mixing and stirring 18 parts ofdeionized water, 0.31 part of “AQUALON KH-10”, 0.03 part ammoniumpersulfate, 5.1 parts methacrylic acid, 5.1 parts 2-hydroxyethylacrylate, 3 parts styrene, 6 parts methyl methacrylate, 1.8 parts ethylacrylate and 9 parts n-butyl acrylate.

Production of Luster Pigment Dispersion for Water-Based Second PigmentedCoating Material (B) Production Example 27

In a stirring/mixing vessel there were uniformly mixed 19 parts of thealuminum pigment paste “GX-180A” (trade name of Asahi Kasei Metals Co.,Ltd., metal content: 74%), 35 parts 2-ethyl-1-hexanol, 8 parts of aphosphate group-containing resin solution(*2) (solid resin content: 4parts) and 0.2 part of 2-(dimethylamino)ethanol, to obtain a lusterpigment dispersion (AL).

(*2): Phosphate Group-Containing Resin Solution:

In a reactor equipped with a thermometer, thermostat, stirrer, refluxcondenser, nitrogen inlet tube and dropper there was placed a mixedsolvent of 27.5 parts of methoxypropanol and 27.5 parts of isobutanol,the mixture was heated to 110° C., and then 121.5 parts of a mixturecomprising 25 parts of styrene, 27.5 parts of n-butyl methacrylate, 20parts of “Isostearyl Acrylate” (trade name of Osaka Organic ChemicalIndustry, Ltd., branched higher alkyl acrylate), 7.5 parts of4-hydroxybutyl acrylate, 15 parts of a phosphate group-containingpolymerizable monomer (*3), 12.5 parts of 2-methacryloyloxyethyl acidphosphate, 10 parts of isobutanol and 4 parts of tert-butylperoxyoctanoate was added to the mixed solvent over a period of 4 hours,and a mixture comprising 0.5 part of tert-butyl peroxyoctanoate and 20parts of isopropanol was further added dropwise over a period of 1 hour.The mixture was then aged while stirring for 1 hour to obtain aphosphate group-containing resin solution with a solid concentration of50%. The acid value due to the phosphate groups of the resin was 83mgKOH/g, the hydroxyl value was 29 mgKOH/g and the weight-averagemolecular weight was 10,000.

(*3) Phosphate Group-Containing Polymerizable Monomer:

After placing 57.5 parts of monobutylphosphoric acid and 41 parts ofisobutanol in a reactor equipped with a thermometer, thermostat,stirrer, reflux condenser, nitrogen inlet tube and dropper and raisingthe temperature to 90° C., 42.5 parts of glycidyl methacrylate was addeddropwise over a period of 2 hours, and then the mixture was stirred for1 hour and aged. Next, 59 parts of isopropanol was added to obtain aphosphate group-containing polymerizable monomer solution with a solidconcentration of 50%. The acid value due to the phosphate groups in theobtained monomer was 285 mgKOH/g.

Production of Polyester Resin for Water-Based Second Pigmented CoatingMaterial (B) Production Example 28

After charging 109 parts of trimethylolpropane, 141 parts of1,6-hexanediol, 126 parts of hexahydrophthalic anhydride and 120 partsof adipic acid into a reactor equipped with a thermometer, thermostat,stirrer, reflux condenser and water separator, and heating between 160°C. and 230° C. for a period of 3 hours, condensation reaction wasconducted at 230° C. for 4 hours. Next, 38.3 parts of trimelliticanhydride was further added for addition of carboxyl groups to theobtained condensation reaction product, reaction was conducted at 170°C. for 30 minutes, and then dilution was performed with2-ethyl-1-hexanol (mass dissolving in 100 g of water at 20° C.: 0.1 g)and 2-(dimethylamino)ethanol was added at 0.5 equivalent with respect tothe acid groups, to obtain a polyester resin (PE) solution with a solidconcentration of 70%. The obtained polyester resin (PE) had an acidvalue of 46 mgKOH/g, a hydroxyl value of 150 mgKOH/g and aweight-average molecular weight of 6,400.

Production of Water-Based Second Pigmented Coating Material (B)Production Example 29

There were uniformly mixed 130 parts of the acrylic resin emulsion (AC)dispersion obtained in Production Example 26 (solid resin content: 39parts), 38.6 parts of the polyester resin solution (PE) solutionobtained in Production Example 28 (solid resin content: 27 parts), 62.2parts of the luster pigment dispersion (AL) obtained in ProductionExample 27 (solid resin content: 4 parts), 35 parts of 2-ethyl-1-hexanoland 37.5 parts of “SAIMEL 325” (trade name of Nihon Cytec IndustriesInc., melamine resin, solid content: 80%) (solid resin content: 30parts), and there were further added “PRIMAL ASE-60” (trade name of Rohm& Haas, polyacrylic acid-based thickening agent),2-(dimethylamino)ethanol and deionized water, to obtain a water-basedsecond pigmented coating material (B-1) having pH 8.0, a solidconcentration of 48% and a viscosity of 60 seconds with a Ford cup No. 4at 20° C.

[Method of Forming Coating Film]

The water-based first pigmented coating materials (A-1) to (A-28)obtained in Examples 1 to 17 and Comparative Examples 1 to 11 and thewater-based second pigmented coating material (B-1) obtained inProduction Example 29 were each used to prepare test sheets in thefollowing manner, and subjected to evaluation testing.

(Fabrication of Article to be Coated for Testing)

A cold-rolled steel sheet that had been subjected to zinc phosphatechemical conversion treatment was electrodeposited with “ELECRON GT-10”(trade name of Kansai Paint Co., Ltd., cationic electrodepositioncoating) to a dry film thickness of 20 μm, and was heated at 170° C. for30 minutes for curing to produce an article to be coated for testing.

Example 18

In a coating environment with a temperature of 23° C. and a humidity of75%, the aforementioned article to be coated for testing and theaforementioned aluminum foil for measurement of the coating filmabsorption percentage of the coating film, were coated with thewater-based first pigmented coating material (A-1) obtained in Example 1to a dry film thickness of 30 μm, using a rotary atomizing coatingmachine. Next, predrying (preheating) was carried out at 80° C. for 5minutes, and then the coating film absorption percentage of thewater-based first pigmented coating film was measured. The measurementresults are shown in Table 3. The water-based second pigmented coatingmaterial (B-1) obtained in Production Example 29 was then applied ontothe water-based first pigmented coating film using a rotary atomizingcoating machine to a dry film thickness of 15 μm, and predrying(preheating) was carried out at 80° C. for 10 minutes. Next, the aqueoussecond pigmented coating film was coated with MAGICRON KINO-1210 (tradename of Kansai Paint Co., Ltd., acrylic resin-based solvent-typeovercoat clear coating material, hereunder also referred to as “clearcoating material (C-1)”) to a dry film thickness of 40 μm, and after itwas allowed to stand for 7 minutes, it was heated at 140° C. for 30minutes, and the water-based first pigmented coating film, aqueoussecond pigmented coating film and clear coating film were simultaneouslybaked to fabricate a test sheet.

Examples 19 to 34, Comparative Examples 12 to 22

A test sheet was fabricated in the same manner as Example 18, exceptthat the water-based first pigmented coating material (A-1) obtained inExample 1 that was used in Example 18 was changed to the water-basedfirst pigmented coating materials (A-2) to (A-28) listed in Table 3below.

Each of the test sheets obtained in Examples 18 to 34 and ComparativeExamples 12 to 22 were evaluated by the following test methods. Theevaluation results are shown in Table 3.

TABLE 3 Water- based Evaluation results first Coating pigmented filmExample coating water Comp. material absorption Distinctness FF WaterChipping Ex. (A) (%) Smoothness of image property resistance resistanceExample A-1 18 11 9 5 VG VG 18 Example A-2 16 11 10 4.5 VG G 19 ExampleA-3 22 12 10 4.5 VG G 20 Example A-4 28 13 12 4.5 VG VG 21 Example A-535 13 12 4.5 G VG 22 Example A-6 17 11 10 4.5 VG G 23 Example A-7 20 1312 4 VG G 24 Example A-8 30 13 12 4.5 G G 25 Example A-9 18 13 12 4.5 VGVG 26 Example A-10 17 13 13 4.5 VG G 27 Example A-11 38 13 13 4 G G 28Example A-12 25 13 13 4 G G 29 Example A-13 29 13 13 4 G G 30 ExampleA-14 37 14 14 4 VG G 31 Example A-15 36 14 14 4 G G 32 Example A-16 2813 11 4 VG G 33 Example A-17 36 14 14 4 G G 34 Comp. A-18 27 18 16 3 F FEx. 12 Comp. A-19 33 13 15 4.5 F F Ex. 13 Comp. A-20 38 14 15 4.5 F FEx. 14 Comp. A-21 38 15 15 4.5 F P Ex. 15 Comp. A-22 48 18 18 3.5 G GEx. 16 Comp. A-23 58 20 21 3 F P Ex. 17 Comp. A-24 17 22 18 4.5 VG G Ex.18 Comp. A-25 18 22 16 4.5 VG F Ex. 19 Comp. A-26 52 12 13 4 P P Ex. 20Comp. A-27 43 12 9 4 F F Ex. 21 Comp. A-28 16 22 18 4.5 VG G Ex. 22

(Test Methods) Coating Film Absorption Percentage (%):

The water-based first pigmented coating material (A) was applied onto analuminum foil for coating film absorption percentage measurement, andthe coating film absorption percentage (%) of the first pigmentedcoating film was calculated by the test method and formula mentionedabove. A smaller coating film absorption percentage indicates lessabsorption of water into the first pigmented coating film upon coatingof the water-based second pigmented coating material (B).

Smoothness:

This was evaluated using the Long Wave (LW) value, measured by a WaveScan (trade name of BYK Gardner). The Long Wave (LW) value is an indexof the amplitude of surface roughness with wavelength of about 600 to1,000 μm, with a smaller measured value representing higher smoothnessof the coating surface.

Distinctness of Image:

This was evaluated using the Short Wave (SW) value, measured by the WaveScan mentioned above. The Short Wave (SW) value is an index of theamplitude of surface roughness with wavelength of about 100 to 600 μm,with a smaller measured value representing higher distinctness of imageof the coating surface.

Flip-Flop Property (FF):

Each test sheet was measured using a MA-68 Multi-AngleSpectrocolorimeter (trade name of X-Rite), determining the L value(brightness) at an acceptance angle of 15° and an acceptance angle of110°, and calculating the FF value by the following formula.

FF value=L value at acceptance angle of 15°/L value at acceptance angleof 110°

A larger FF value indicates greater variation in the L value(brightness) depending on the observation angle (acceptance angle), anda more excellent flip-flop property.

Water Resistance:

The test sheet was immersed for 240 hours in warm water at 40° C. andthen raised and dried at 20° C. for 12 hours, after which the multilayercoating film on the test sheet was notched with a cutter in alattice-like manner reaching to the basis material, to create 100 squaregrids with sizes of 2 mm×2 mm. Pressure-sensitive adhesive cellophanetape was then attached to the surface and the tape was abruptly peeledoff at 20° C., after which the residual state of the square grid coatingfilm was examined. The evaluation standard for the water resistance wasas follows.

VG: 100 of the square grids of the coating film remained, with nochipping of the edges of the square grids.

G: 100 of the square grids of the coating film remained, but withchipping of the edges of the square grids.

F: 90 to 99 of the square grids of the coating film remained.

P: 89 or fewer of the square grids of the coating film remained.

Chipping Resistance:

The test sheet was set on the sample holding stage of a Model JA-400chipping tester (trade name of Suga Test Instruments Co., Ltd., chippingresistance tester), and 50 g of crushed granite of #7 grain size wasimpacted onto the test sheet at an angle of 45°, using compressed air at0.392 MPa (4 kgf/cm²), at −20° C. from a distance of 30 cm. The obtainedtest sheet was then washed with water and dried, and cloth adhesive tape(product of Nichiban Co., Ltd.) was attached to the coating surface andpeeled off, after which the extent of damage in the coating film wasvisually examined and evaluated by the following standard.

VG: Very small damage size, no exposure of electrodeposition surface orbase steel sheet.

G: Small damage size, no exposure of electrodeposition surface or basesteel sheet.

F: Small damage size, but some exposure of electrodeposition surface orbase steel sheet.

P: Considerable damage size, with exposure of base steel sheet.

1. A water-based coating composition to be used as a water-based firstpigmented coating material (A) in a multilayer coating film-formingmethod in which the following steps (1) to (4) are carried out in order:step (1): a step of coating an article to be coated with a water-basedfirst pigmented coating material (A) to form a first pigmented coatingfilm, step (2): a step of coating the uncured first pigmented coatingfilm formed in step (1) with a water-based second pigmented coatingmaterial (B) to form a second pigmented coating film, step (3): a stepof coating the uncured second pigmented coating film formed in step (2)with a clear coating material (C) to form a clear coating film, and step(4): a step of bake curing the multilayer coating film formed in steps(1) to (3), wherein: the water-based coating composition comprises 7 to60 mass % of an acrylic resin emulsion (a), 10 to 60 mass % of a coatingfilm-forming resin (b) and 10 to 50 mass % of a curing agent (c), basedon the total mass of the solid resin content in the water-based coatingcomposition, the acrylic resin emulsion (a) has a core-shell structure,the core section comprising, as a copolymerizing component, 0.1 to 10mass % of a polymerizable unsaturated monomer having two or morepolymerizable unsaturated groups in the molecule, based on the totalmass of the monomers of the core section, and the hydroxyl value of thecore section is 50 to 200 mgKOH/g and the hydroxyl value of the shellsection is 50 to 200 mgKOH/g.
 2. A water-based coating compositionaccording to claim 1, wherein the core section comprises, as acopolymerizing component, 30 to 90 mass % of a copolymerizableunsaturated monomer with a C4 or greater hydrocarbon group, based on thetotal mass of the monomers of the core section, the acid value of theacrylic resin emulsion (a) is 5 to 25 mgKOH/g, and its glass transitiontemperature (Tg) is 20° C. or higher.
 3. A water-based coatingcomposition according to claim 1, wherein the coating film-forming resin(b) comprises at least one type selected from among polyester resins(b1), water-soluble acrylic resins (b2) and urethane resins (b3).
 4. Awater-based coating composition according to claim 1, wherein the curingagent (c) comprises an amino resin (c1) and/or a blocked polyisocyanatecompound (c2), and the water-based coating composition includes 5 to 30mass % of an amino resin (c1) and/or 5 to 30 mass % of a blockedpolyisocyanate compound (c2), based on the total mass of the solid resincontent in the water-based coating composition.
 5. A water-based coatingcomposition according to claim 1, wherein the blocked polyisocyanatecompound (c2) comprises an active methylene-type blocked polyisocyanatecompound (c3) with at least one type of blocked isocyanate groupselected from the group consisting of blocked isocyanate groupsrepresented by the following formula (I):

(wherein R¹, R², R⁴ and R⁵ each independently represent a C1-12hydrocarbon group and R³ represents a C1-12 straight-chain or branchedalkylene group), blocked isocyanate groups represented by the followingformula (II):

(wherein R², R³, R⁴ and R⁵ have the same definitions as above), andblocked isocyanate groups represented by the following formula (III):

(wherein R², R³, R⁴ and R⁵ have the same definitions as above, and R⁶represents a C1-12 hydrocarbon group).
 6. A water-based coatingcomposition according to claim 1, wherein the water-based coatingcomposition includes, as a pigment, titanium dioxide and/or bariumsulfate in a total range of 50 to 150 mass % based on the total mass ofthe solid resin content.
 7. A multilayer coating film-forming methodcomprising the following steps (1) to (4): step (1): a step of coatingan article to be coated with a water-based first pigmented coatingmaterial (A) to form a first pigmented coating film, step (2): a step ofcoating the uncured first pigmented coating film formed in step (1) witha water-based second pigmented coating material (B) to form a secondpigmented coating film, step (3): a step of coating the uncured secondpigmented coating film formed in step (2) with a clear coating material(C) to form a clear coating film, and step (4): a step of bake curingthe multilayer coating film formed in steps (1) to (3), wherein thewater-based coating composition as the water-based first pigmentedcoating material (A) comprises 7 to 60 mass % of an acrylic resinemulsion (a), 10 to 60 mass % of a coating film-forming resin (b) and 10to 50 mass % of a curing agent (c), based on the total mass of the solidresin content in the water-based coating composition, the acrylic resinemulsion (a) has a core-shell structure, the core section comprising, asa copolymerizing component, 0.1 to 10 mass % of a polymerizableunsaturated monomer having two or more polymerizable unsaturated groupsin the molecule, based on the total mass of the monomers of the coresection, and the hydroxyl value of the core section is 50 to 200 mgKOH/gand the hydroxyl value of the shell section is 50 to 200 mgKOH/g.
 8. Themethod according to claim 7, wherein the core section comprises, as acopolymerizing component, 30 to 90 mass % of a copolymerizableunsaturated monomer with a C4 or greater hydrocarbon group, based on thetotal mass of the monomers of the core section, the acid value of theacrylic resin emulsion (a) is no greater than 15 mgKOH/g, and its glasstransition temperature (Tg) is 20° C. or higher.
 9. An article with amultilayer coating film, formed by the method according to claim 7.